why did jj thomson experiment with cathode ray tubes

Discovering the electron: JJ Thomson and the Cathode Ray Tube

Concept Introduction: JJ Thomson and the Discovery of the Electron

The discovery of the electron was an important step for physics, chemistry, and all fields of science. JJ Thomson made the discovery using the cathode ray tube. Learn all about the discovery, the importance of the discovery, and JJ Thomson in this tutorial article.

Further Reading on the Electron

Electron Orbital and Electron Shapes Writing Electron Configurations Electron Shells What are valence electrons? Electron Affinity Aufbau Principle

Who was JJ Thomson?

JJ Thomson was an English physicist who is credited with discovery of the electron in 1897. Thompson was born in December 1856 in Manchester, England and was educated at the University of Manchester and then the University of Cambridge, graduating with a degree in mathematics. Thompson made the switch to physics a few years later and began studying the properties of cathode rays. In addition to this work, Thomson also performed the first-ever mass spectrometr y experiments, discovered the first isotope and made important contributions both to the understanding of positively charged particles and electrical conductivity in gases.

Thomson did most of this work while leading the famed Cavendish Laboratory at the University of Cambridge. Although he received the Nobel Prize in physics and not chemistry, Thomson’s contributions to the field of chemistry are numerous. For instance, the discovery of the electron was vital to the development of chemistry today, and it was the first subatomic particle to be discovered. The proton and the neutron would soon follow as the full structure of the atom was discovered.

What is a cathode ray tube and why was it important?

Prior to the discovery of the electron, several scientists suggested that atoms consisted of smaller pieces. Yet until Thomson, no one had determined what these might be. Cathode rays played a critical role in unlocking this mystery. Thomson determined that charged particles much lighter than atoms , particles that we now call electrons made up cathode rays. Cathode rays form when electrons emit from one electrode and travel to another. The transfer occurs due to the application of a voltage in vacuum. Thomson also determined the mass to charge ratio of the electron using a cathode ray tube, another significant discovery.

How did Thomson make these discoveries?

Thomson was able to deflect the cathode ray towards a positively charged plate deduce that the particles in the beam were negatively charged. Then Thomson measured how much various strengths of magnetic fields bent the particles. Using this information Thomson determined the mass to charge ratio of an electron. These were the two critical pieces of information that lead to the discovery of the electron. Thomson was now able to determine that the particles in question were much smaller than atoms, but still highly charged. He finally proved atoms consisted of smaller components, something scientists puzzled over for a long time. Thomson called the particle “corpuscles” , not an electron. George Francis Fitzgerald suggested the name electron.

Why was the discovery of the electron important?

The discovery of the electron was the first step in a long journey towards a better understanding of the atom and chemical bonding. Although Thomson didn’t know it, the electron would turn out to be one of the most important particles in chemistry. We now know the electron forms the basis of all chemical bonds. In turn chemical bonds are essential to the reactions taking place around us every day. Thomson’s work provided the foundation for the work done by many other important scientists such as Einstein, Schrodinger, and Feynman.

Interesting Facts about JJ Thomson

Not only did Thomson receive the Nobel Prize in physics in 1906 , but his son Sir George Paget Thomson won the prize in 1937. A year earlier, in 1936, Thomson wrote an autobiography called “Recollections and Reflections”. He died in 1940, buried near Isaac Newton and Charles Darwin. JJ stands for “Joseph John”. Strangely, another author with the name JJ Thomson wrote a book with the same name in 1975. Thomson had many famous students, including Ernest Rutherford.

Discovery of the Electron: Further Reading

Protons, Neutrons & Electrons Discovering the nucleus with gold foil Millikan oil drop experiment Phase Diagrams

JJ Thompson’s Discovery of Electron: Cathode Ray Tube Experiment Explained

JJ Thomson discovered the electron in 1897 and there are tons of videos about it.  However, most videos miss what JJ Thomson himself said was the motivating factor: a debate about how cathode rays move.  Want to know not only how but why electrons were discovered?

Table of Contents

The start of jj thomson, how thomson discovered electrons: trials and errors, thomson’s conclusion.

A short history of Thomson: Joseph John Thomson, JJ on papers, to friends, and even to his own son [1] , was born in Lancashire, England to a middle class bookseller.  When he was 14 years old, Thomson planned to get an apprenticeship to a locomotive engineer but it had a long waiting list, so, he applied to and was accepted at that very young age to Owen’s college. 

Thompson later recalled that, “the authorities at Owens College thought my admission was such a scandal – I expect they feared that students would soon be coming in perambulators  – that they passed regulations raising the minimum age for admission, so that such a catastrophe should not happen again.

[2] ”  While in school, his father died, and his family didn’t have enough money for the apprenticeship.  Instead, he relied on scholarships at universities – ironically leading him to much greater fame in academia. In 1884, at the tender age of 28, Thomson applied to be the head of the Cavendish Research Institute. 

He mostly applied as a lark and was as surprised as anyone to actually get the position!  “I felt like a fisherman who…had casually cast a line in an unlikely spot and hooked a fish much too heavy for him to land. [3] ”  Suddenly, he had incredible resources, stability and ability to research whatever he wished. 

He ended up having an unerring ability to pinpoint interesting phenomena for himself and for others. In fact, a full eight of his research assistants and his son eventually earned Nobel Prizes, but, of course, like Thomson’s own Nobel Prize, that was in the future.

Why did J. J. Thomson discover the electron in 1897?  Well, according to Thomson: “the discovery of the electron began with an attempt to explain the discrepancy between the behavior of cathode rays under magnetic and electric forces [4] .”  What did he mean by that? 

Well, a cathode ray, or a ray in a vacuum tube that emanates from the negative electrode, can be easily moved with a magnet.  This gave a charismatic English chemist named William Crookes the crazy idea that the cathode ray was made of charged particles in 1879! 

However, 5 years later, a young German scientist named Heinrich Hertz found that he could not get the beam to move with parallel plates, or with an electric field.  Hertz decided that Crookes was wrong, if the cathode ray was made of charged particles then it should be attracted to a positive plate and repulsed from a negative plate. 

Ergo, it couldn’t be particles, and Hertz decided it was probably some new kind of electromagnetic wave, like a new kind of ultraviolet light.  Further, in 1892, Hertz accidentally discovered that cathode rays could tunnel through thin pieces of metal, which seemed like further proof that Crookes was so very wrong.

Then, in December of 1895, a French physicist named Jean Perrin used a magnet to direct a cathode ray into and out of an electroscope (called a Faraday cylinder) and measured its charge.  Perrin wrote, “the Faraday cylinder became negatively charged when the cathode rays entered it, and only when they entered it; the cathode rays are thus charged with negative electricity .

[5] ”  This is why JJ Thomson was so confused, he felt that Perrin had, “conclusive evidence that the rays carried a charge of negative electricity” except that, “Hertz found that when they were exposed to an electric force they were not deflected at all.”  What was going on?

In 1896, Thomson wondered if there might have been something wrong with Hertz’s experiment with the two plates.  Thomson knew that the cathode ray tubes that they had only work if there is a little air in the tube and the amount of air needed depended on the shape of the terminals.

Thomson wondered if the air affected the results.  Through trial and error, Thomson found he could get a “stronger” beam by shooting it through a positive anode with a hole in it.  With this system he could evacuate the tube to a much higher degree and, if the vacuum was good enough, the cathode ray was moved by electrically charged plates, “just as negatively electrified particles would be.

[6] ” (If you are wondering why the air affected it, the air became ionized in the high electric field and became conductive.  The conductive air then acted like a Faraday cage shielding the beam from the electric field.)

As stated before, Heinrich Hertz also found that cathode rays could travel through thin solids.  How could a particle do that?  Thomson thought that maybe particles could go through a solid if they were moving really, really fast.  But how to determine how fast a ray was moving? 

Thomson made an electromagnetic gauntlet.  First, Thomson put a magnet near the ray to deflect the ray one-way and plates with electric charge to deflect the ray the other way.  He then added or reduced the charge on the plates so that the forces were balanced and the ray went in a straight line. 

He knew that the force from the magnet depended on the charge of the particle, its speed and the magnetic field (given the letter B).  He also knew that the electric force from the plates only depended on the charge of the particle and the Electric field.  Since these forces were balanced, Thomson could determine the speed of the particles from the ratio of the two fields. 

Thomson found speeds as big as 60,000 miles per second or almost one third of the speed of light.  Thomson recalled, “In all cases when the cathode rays are produced their velocity is much greater than the velocity of any other moving body with which we are acquainted. [7] ”  

Thomson then did something even more ingenious; he removed the magnetic field.  Now, he had a beam of particles moving at a known speed with a single force on them.  They would fall, as Thomson said, “like a bullet projected horizontally with a velocity v and falling under gravity [8] ”.  

Note that these “bullets” are falling because of the force between their charge and the charges on the electric plates as gravity is too small on such light objects to be influential.  By measuring the distance the bullets went he could determine the time they were in the tube and by the distance they “fell” Thomson could determine their acceleration. 

Using F=ma Thomson determine the ratio of the charge on the particle to the mass (or e/m).  He found some very interesting results.  First, no matter what variables he changed in the experiment, the value of e/m was constant.  “We may… use any kind of substance we please for the electrodes and fill the tube with gas of any kind and yet the value of e/m will remain the same.

[9] ”  This was a revolutionary result.  Thomson concluded that everything contained these tiny little things that he called corpuscles (and we call electrons).  He also deduced that the “corpuscles” in one item are exactly the same as the “corpuscles” in another.  So, for example, an oxygen molecule contains the same kind of electrons as a piece of gold!  Atoms are the building blocks of matter but inside the atoms (called subatomic) are these tiny electrons that are the same for everything .

The other result he found was that the value of e/m was gigantic, 1,700 times bigger than the value for a charged Hydrogen atom, the object with the largest value of e/m before this experiment.   So, either the “corpuscle” had a ridiculously large charge or it was, well, ridiculously small.   

A student of Thomson’s named C. T. R. Wilson had experimented with slowly falling water droplets that found that the charge on the corpuscles were, to the accuracy of the experiment, the same as the charge on a charged Hydrogen atom!   Thomson concluded that his corpuscles were just very, very, tiny, about 1,700 times smaller then the Hydrogen atom [1] .  These experiments lead Thomson to come to some interesting conclusions:

• Electrons are in everything and are well over a thousand times smaller then even the smallest atom. 
• Benjamin Franklin thought positive objects had too much “electrical fire” and negative had too little.  Really, positive objects have too few electrons and negative have too many.  Oops.
• Although since Franklin, people thought current flowed from the positive side to the negative, really, the electrons are flowing the other way.  When a person talks about “current” that flows from positive to negative they are talking about something that is not real!   True “electric current” flows from negative to positive and is the real way the electrons move. [although by the time that people believed J.J. Thomson, it was too late to change our electronics, so people just decided to stick with “current” going the wrong way!]
• Since electrons are tiny and in everything but most things have a neutral charge, and because solid objects are solid, the electrons must be swimming in a sea or soup of positive charges.  Like raisons in a raison cookie.

The first three are still considered correct over one hundred years later.  The forth theory, the “plum pudding model” named after a truly English “desert” with raisins in sweet bread that the English torture people with during Christmas, was proposed by Thomson in 1904. 

In 1908, a former student of Thomson’snamed Ernest Rutherford was experimenting with radiation, and inadvertently demolished the “plum pudding model” in the process.  However, before I can get into Rutherford’s gold foil experiment, I first want to talk about what was going on in France concurrent to Thomson’s experiments. 

This is a story of how a new mother working mostly in a converted shed discovered and named the radium that Rutherford was experimenting with.  That woman’s name was Marie Sklodowska Curie, and that story is next time on the Lightning Tamers.

[1] the current number is 1,836 but Thomson got pretty close

[1] p 14 “Flash of the Cathode Rays: A History of JJ Thomson’s Electron” Dahl

[2] Thompson, J.J. Recollections and Reflections p. 2 Referred to in Davis & Falconer JJ. Thompson and the Discovery of the Electron 2002 p. 3

[3] Thomson, Joseph John Recollections and Reflections p. 98 quoted in Davis, E.A & Falconer, Isabel JJ Thomson and the Discovery of the Electron 2002 p. 35

[4]   Thomson, JJ Recollections and Reflections p. 332-3

[5] “New Experiments on the Kathode Rays” Jean Perrin, December 30, 1985 translation appeared in Nature, Volume 53, p 298-9, January 30, 1896

[6] Nobel Prize speech?

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• Structure of Atom

Cathode Ray Experiment

What is cathode ray tube.

A cathode-ray tube (CRT) is a vacuum tube in which an electron beam, deflected by applied electric or magnetic fields, produces a trace on a fluorescent screen.

The function of the cathode ray tube is to convert an electrical signal into a visual display. Cathode rays or streams of electron particles are quite easy to produce, electrons orbit every atom and move from atom to atom as an electric current.

Table of Contents

Cathode ray tube, recommended videos.

• J.J.Thomson Experiment

Apparatus Setup

Procedure of the experiment.

• Frequently Asked Questions – FAQs

In a cathode ray tube, electrons are accelerated from one end of the tube to the other using an electric field. When the electrons hit the far end of the tube they give up all the energy they carry due to their speed and this is changed to other forms such as heat. A small amount of energy is transformed into X-rays.

The cathode ray tube (CRT), invented in 1897 by the German physicist Karl Ferdinand Braun, is an evacuated glass envelope containing an electron gun a source of electrons and a fluorescent light, usually with internal or external means to accelerate and redirect the electrons. Light is produced when electrons hit a fluorescent tube.

The electron beam is deflected and modulated in a manner that allows an image to appear on the projector. The picture may reflect electrical wave forms (oscilloscope), photographs (television, computer monitor), echoes of radar-detected aircraft, and so on. The single electron beam can be processed to show movable images in natural colours.

J. J. Thomson Experiment – The Discovery of Electron

The Cathode ray experiment was a result of English physicists named J. J. Thomson experimenting with cathode ray tubes. During his experiment he discovered electrons and it is one of the most important discoveries in the history of physics. He was even awarded a Nobel Prize in physics for this discovery and his work on the conduction of electricity in gases.

However, talking about the experiment, J. J. Thomson took a tube made of glass containing two pieces of metal as an electrode. The air inside the chamber was subjected to high voltage and electricity flowing through the air from the negative electrode to the positive electrode.

J. J. Thomson designed a glass tube that was partly evacuated, i.e. all the air had been drained out of the building. He then applied a high electric voltage at either end of the tube between two electrodes. He observed a particle stream (ray) coming out of the negatively charged electrode (cathode) to the positively charged electrode (anode). This ray is called a cathode ray and is called a cathode ray tube for the entire construction.

The experiment Cathode Ray Tube (CRT) conducted by J. J. Thomson, is one of the most well-known physical experiments that led to electron discovery . In addition, the experiment could describe characteristic properties, in essence, its affinity to positive charge, and its charge to mass ratio. This paper describes how J is simulated. J. Thomson experimented with Cathode Ray Tube.

The major contribution of this work is the new approach to modelling this experiment, using the equations of physical laws to describe the electrons’ motion with a great deal of accuracy and precision. The user can manipulate and record the movement of the electrons by assigning various values to the experimental parameters.

A Diagram of JJ.Thomson Cathode Ray Tube Experiment showing Electron Beam – A cathode-ray tube (CRT) is a large, sealed glass tube.

The apparatus of the experiment incorporated a tube made of glass containing two pieces of metals at the opposite ends which acted as an electrode. The two metal pieces were connected with an external voltage. The pressure of the gas inside the tube was lowered by evacuating the air.

• Apparatus is set up by providing a high voltage source and evacuating the air to maintain the low pressure inside the tube.
• High voltage is passed to the two metal pieces to ionize the air and make it a conductor of electricity.
• The electricity starts flowing as the circuit was complete.
• To identify the constituents of the ray produced by applying a high voltage to the tube, the dipole was set up as an add-on in the experiment.
• The positive pole and negative pole were kept on either side of the discharge ray.
• When the dipoles were applied, the ray was repelled by the negative pole and it was deflected towards the positive pole.
• This was further confirmed by placing the phosphorescent substance at the end of the discharge ray. It glows when hit by a discharge ray. By carefully observing the places where fluorescence was observed, it was noted that the deflections were on the positive side. So the constituents of the discharge tube were negatively charged.

After completing the experiment J.J. Thomson concluded that rays were and are basically negatively charged particles present or moving around in a set of a positive charge. This theory further helped physicists in understanding the structure of an atom . And the significant observation that he made was that the characteristics of cathode rays or electrons did not depend on the material of electrodes or the nature of the gas present in the cathode ray tube. All in all, from all this we learn that the electrons are in fact the basic constituent of all the atoms.

Most of the mass of the atom and all of its positive charge are contained in a small nucleus, called a nucleus. The particle which is positively charged is called a proton. The greater part of an atom’s volume is empty space.

The number of electrons that are dispersed outside the nucleus is the same as the number of positively charged protons in the nucleus. This explains the electrical neutrality of an atom as a whole.

Uses of Cathode Ray Tube

• Used as a most popular television (TV) display.
• X-rays are produced when fast-moving cathode rays are stopped suddenly.
• The screen of a cathode ray oscilloscope, and the monitor of a computer, are coated with fluorescent substances. When the cathode rays fall off the screen pictures are visible on the screen.

Frequently Asked Questions – FAQs

What are cathode ray tubes made of.

The cathode, or the emitter of electrons, is made of a caesium alloy. For many electronic vacuum tube systems, Cesium is used as a cathode, as it releases electrons readily when heated or hit by light.

Where can you find a cathode ray tube?

Cathode rays are streams of electrons observed in vacuum tubes (also called an electron beam or an e-beam). If an evacuated glass tube is fitted with two electrodes and a voltage is applied, it is observed that the glass opposite the negative electrode glows from the electrons emitted from the cathode.

How did JJ Thomson find the electron?

In the year 1897 J.J. Thomson invented the electron by playing with a tube that was Crookes, or cathode ray. He had shown that the cathode rays were charged negatively. Thomson realized that the accepted model of an atom did not account for the particles charged negatively or positively.

What are the properties of cathode rays?

They are formed in an evacuated tube via the negative electrode, or cathode, and move toward the anode. They journey straight and cast sharp shadows. They’ve got strength, and they can do the job. Electric and magnetic fields block them, and they have a negative charge.

What do you mean by cathode?

A device’s anode is the terminal on which current flows in from outside. A device’s cathode is the terminal from which current flows out. By present, we mean the traditional positive moment. Because electrons are charged negatively, positive current flowing in is the same as outflowing electrons.

Who discovered the cathode rays?

Studies of cathode-ray began in 1854 when the vacuum tube was improved by Heinrich Geissler, a glassblower and technical assistant to the German physicist Julius Plücker. In 1858, Plücker discovered cathode rays by sealing two electrodes inside the tube, evacuating the air and forcing it between the electrode’s electric current.

Which gas is used in the cathode ray experiment?

For better results in a cathode tube experiment, an evacuated (low pressure) tube is filled with hydrogen gas that is the lightest gas (maybe the lightest element) on ionization, giving the maximum charge value to the mass ratio (e / m ratio = 1.76 x 10 ^ 11 coulombs per kg).

What is the Colour of the cathode ray?

Cathode-ray tube (CRT), a vacuum tube which produces images when electron beams strike its phosphorescent surface. CRTs can be monochrome (using one electron gun) or coloured (using usually three electron guns to produce red, green, and blue images that render a multicoloured image when combined).

How cathode rays are formed?

Cathode rays come from the cathode because the cathode is charged negatively. So those rays strike and ionize the gas sample inside the container. The electrons that were ejected from gas ionization travel to the anode. These rays are electrons that are actually produced from the gas ionization inside the tube.

What are cathode rays made of?

Thomson showed that cathode rays were composed of a negatively charged particle, previously unknown, which was later named electron. To render an image on a screen, Cathode ray tubes (CRTs) use a focused beam of electrons deflected by electrical or magnetic fields.

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Cathode Ray Experiment

The electric experiment by j.j. thomson.

J. J. Thomson was one of the great scientists of the 19th century; his inspired and innovative cathode ray experiment greatly contributed to our understanding of the modern world.

This article is a part of the guide:

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• Physics Experiments
• Brownian Movement

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• 1 Physics Experiments
• 2 Ben Franklin Kite
• 3 Brownian Movement
• 4 Cathode Ray Experiment

Like most scientists of that era, he inspired generations of later physicists, from Einstein to Hawking .

His better-known research proved the existence of negatively charged particles, later called electrons, and earned him a deserved Nobel Prize for physics. This research led to further experiments by Bohr and Rutherford, leading to an understanding of the structure of the atom.

What is a Cathode Ray Tube?

Even without consciously realizing it, most of us are already aware of what a cathode ray tube is.

Look at any glowing neon sign or any ‘old-fashioned’ television set, and you are looking at the modern descendants of the cathode ray tube.

Physicists in the 19th century found out that if they constructed a glass tube with wires inserted in both ends, and pumped out as much of the air as they could, an electric charge passed across the tube from the wires would create a fluorescent glow. This cathode ray also became known as an ‘electron gun’.

Later and improved cathode ray experiments found that certain types of glass produced a fluorescent glow at the positive end of the tube. William Crookes discovered that a tube coated in a fluorescing material at the positive end, would produce a focused ‘dot’ when rays from the electron gun hit it.

With more experimentation, researchers found that the ‘cathode rays’ emitted from the cathode could not move around solid objects and so traveled in straight lines, a property of waves. However, other researchers, notably Crookes, argued that the focused nature of the beam meant that they had to be particles.

Physicists knew that the ray carried a negative charge but were not sure whether the charge could be separated from the ray. They debated whether the rays were waves or particles, as they seemed to exhibit some of the properties of both. In response, J. J. Thomson constructed some elegant experiments to find a definitive and comprehensive answer about the nature of cathode rays.

Thomson’s First Cathode Ray Experiment

Thomson had an inkling that the ‘rays’ emitted from the electron gun were inseparable from the latent charge, and decided to try and prove this by using a magnetic field.

His first experiment was to build a cathode ray tube with a metal cylinder on the end. This cylinder had two slits in it, leading to electrometers, which could measure small electric charges.

He found that by applying a magnetic field across the tube, there was no activity recorded by the electrometers and so the charge had been bent away by the magnet. This proved that the negative charge and the ray were inseparable and intertwined.

Thomson's Cathode Ray Second Experiment

Like all great scientists, he did not stop there, and developed the second stage of the experiment, to prove that the rays carried a negative charge. To prove this hypothesis, he attempted to deflect them with an electric field.

Earlier experiments had failed to back this up, but Thomson thought that the vacuum in the tube was not good enough, and found ways to improve greatly the quality.

For this, he constructed a slightly different cathode ray tube, with a fluorescent coating at one end and a near perfect vacuum. Halfway down the tube were two electric plates, producing a positive anode and a negative cathode, which he hoped would deflect the rays.

As he expected, the rays were deflected by the electric charge, proving beyond doubt that the rays were made up of charged particles carrying a negative charge. This result was a major discovery in itself, but Thomson resolved to understand more about the nature of these particles.

Thomson's Third Experiment

The third experiment was a brilliant piece of scientific deduction and shows how a series of experiments can gradually uncover truths.

Many great scientific discoveries involve performing a series of interconnected experiments, gradually accumulating data and proving a hypothesis .

He decided to try to work out the nature of the particles. They were too small to have their mass or charge calculated directly, but he attempted to deduce this from how much the particles were bent by electrical currents, of varying strengths.

Thomson found out that the charge to mass ratio was so large that the particles either carried a huge charge, or were a thousand times smaller than a hydrogen ion. He decided upon the latter and came up with the idea that the cathode rays were made of particles that emanated from within the atoms themselves, a very bold and innovative idea.

Later Developments

Thomson came up with the initial idea for the structure of the atom, postulating that it consisted of these negatively charged particles swimming in a sea of positive charge. His pupil, Rutherford, developed the idea and came up with the theory that the atom consisted of a positively charged nucleus surrounded by orbiting tiny negative particles, which he called electrons.

Quantum physics has shown things to be a little more complex than this but all quantum physicists owe their legacy to Thomson. Although atoms were known about, as apparently indivisible elementary particles, he was the first to postulate that they had a complicated internal structure.

Thomson's greatest gift to physics was not his experiments, but the next generation of great scientists who studied under him, including Rutherford, Oppenheimer and Aston. These great minds were inspired by him, marking him out as one of the grandfathers of modern physics.

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Cathode Ray Tube experiment of J.J. Thomson

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This wiki is incomplete.

J.J. Thompson performed a brilliant experiment which proved that atom consisted of charged subatomic particle(s). Before his work, atom was considered to be indestructible. However, when atoms were studied in large electric fields, evidences came up indicating that they consisted of subatomic particles.

The experiment

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Subatomic science: JJ Thomson's discovery of the electron

Read about how JJ Thomson announced his discovery of the electron at the Royal Institution in this blog by our Head of Heritage and Collections. 

JJ Thomson, while familiar to scientists, is not necessarily a name most people would recognise; however, anyone who has undertaken any science at school will have heard of an electron.

It is Thomson we have to thank for discovering this fundamental breakthrough in science and announcing his discovery to the world during a lecture here at the Royal Institution in 1897.

What is an electron?

The technical definition is:

"An electron is a stable subatomic particle with a negative electrical charge. Unlike protons and neutrons, electrons are not constructed from even smaller components."

As a non-scientist this definition is something I have heard before but must confess is not something that means a great deal to me. It is an explanation in its basic form but doesn’t convey really what an electron is or what the impact of its discovery made

John Dalton's atomic theory

Prior to 1897, scientists had hypothesised about the makeup of the universe at the atomic and subatomic level but had not been able to prove any theories. The atom had been known about for many years.

In 1808, chemist John Dalton developed an argument that led to a realisation: that perhaps all matter, the things or objects that make up the universe are made of tiny, little bits.

These are fundamental and indivisible bits and named after the ancient Greek words ‘a’ meaning not and ‘tomos’ meaning cut therefore ‘atomos’ or uncuttable. Atoms.

JJ Thomson's cathode ray tube experiments

Thomson, a highly respected theoretical physics professor at Cambridge University, undertook a series of experiments designed to study the nature of electric discharge in a high-vacuum cathode-ray tube – he was attempting to solve a long-standing controversy regarding the nature of cathode rays, which occur when an electric current is driven through a vessel from which most of the air or other gas has been pumped out.

This was something that many scientists were investigating at the time. It was Thomson that made the breakthrough however, concluding through his experimentation that particles making up the rays were 1,000 times lighter than the lightest atom, proving that something smaller than atoms existed.

Thomson likened the composition of atoms to plum pudding, with negatively-charged ‘corpuscles’ dotted throughout a positively charged field.

G Johnstone Stoney coins the term 'electron'

Thomson explained within his lecture all of his experiments and the results, never mentioning the word electron but instead sticking to corpuscles to explain these tiny particles in the same terms as biological cells (corpuscles are a minute body or cell in an organism).

Such would they have remained if not for the term 'electron' coined by G Johnstone Stoney who in 1891 denoted the unit of charge found in experiments that passed electrical current through chemicals.

It was then in 1897 after Thomson’s publication of his research that Irish physicist George Francis Fitzgerald suggested that the term be applied to Thomson's research instead of corpuscles to better describe these newly discovered subatomic particles.

JJ Thomson and the Royal Institution

Thomson had a long-standing relationship with the Royal Institution during his long academic career in Cambridge, lecturing many times on the development of physics through Discourses and educational lectures to all ages.

Thomson was a great friend of Sir William Henry Bragg and Sir William Lawrence Bragg, who jointly won the Nobel Prize in 1915 for the development of x-ray crystallography, and who were both former Director’s of the Royal Institution.

JJ Thomson's Nobel Prize

Thomson received the Nobel Prize for his work in Physics in 1906 and was knighted in 1908. The studies of nuclear organisation that continue even to this day and the further identification of elementary particles have all followed the accomplishments of Thomson and his discovery in 1897.

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Key Questions

Thomson's experiments with cathode ray tubes helped him to discover the electron.

This ushered in a model of atomic structure referred to as the plum pudding model. I like to think of it like a sphere shaped chocolate chip cookie since plum pudding is not super popular in the US.

The cookie dough (they didn't know what it was yet) is positively charged and the chocolate chips (electrons) are negatively charged and scattered randomly throughout the cookie (atom). The positive and negative charges cancel producing a neutral atom.

• Scientific Biographies

Joseph John “J. J.” Thomson

In 1897 Thomson discovered the electron and then went on to propose a model for the structure of the atom. His work also led to the invention of the mass spectrograph.

The British physicist Joseph John “J. J.” Thomson (1856–1940) performed a series of experiments in 1897 designed to study the nature of electric discharge in a high-vacuum cathode-ray tube, an area being investigated by many scientists at the time.

Thomson interpreted the deflection of the rays by electrically charged plates and magnets as evidence of “bodies much smaller than atoms” (electrons) that he calculated as having a very large value for the charge-to-mass ratio. Later he estimated the value of the charge itself.

Structure of the Atom and Mass Spectrography

In 1904 Thomson suggested a model of the atom as a sphere of positive matter in which electrons are positioned by electrostatic forces. His efforts to estimate the number of electrons in an atom from measurements of the scattering of light, X, beta, and gamma rays initiated the research trajectory along which his student Ernest Rutherford moved.

Thomson’s last important experimental program focused on determining the nature of positively charged particles. Here his techniques led to the development of the mass spectrograph. His assistant, Francis Aston, developed Thomson’s instrument further and with the improved version was able to discover isotopes—atoms of the same element with different atomic weights—in a large number of nonradioactive elements.

Early Life and Education

Ironically, Thomson—great scientist and physics mentor—became a physicist by default. His father intended him to be an engineer, which in those days required an apprenticeship, but his family could not raise the necessary fee. Instead young Thomson attended Owens College, Manchester, which had an excellent science faculty. He was then recommended to Trinity College, Cambridge, where he became a mathematical physicist.

In 1884 he was named to the prestigious Cavendish Professorship of Experimental Physics at Cambridge, although he had personally done very little experimental work. Even though he was clumsy with his hands, he had a genius for designing apparatus and diagnosing its problems. He was a good lecturer, encouraged his students, and devoted considerable attention to the wider problems of science teaching at university and secondary levels.

Ties to the Chemical Community

Of all the physicists associated with determining the structure of the atom, Thomson remained most closely aligned to the chemical community. His nonmathematical atomic theory—unlike early quantum theory—could also be used to account for chemical bonding and molecular structure (see Gilbert Newton Lewis and Irving Langmuir ). In 1913 Thomson published an influential monograph urging chemists to use the mass spectrograph in their analyses.

A Nobel Prize

Thomson received various honors, including the Nobel Prize in Physics in 1906 and a knighthood in 1908. He also had the great pleasure of seeing several of his close associates receive their own Nobel Prizes, including Rutherford in chemistry (1908) and Aston in chemistry (1922).

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1.6: The Discovery of the Electron

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Learning Objectives

• To become familiar with the feature of an electron.
• Summarize and interpret the results of the experiments of Thomson and Millikan.

Long before the end of the 19th century, it was well known that applying a high voltage to a gas contained at low pressure in a sealed tube (called a gas discharge tube) caused electricity to flow through the gas, which then emitted light (Figure \(\PageIndex{1}\)). Researchers trying to understand this phenomenon found that an unusual form of energy was also emitted from the cathode, or negatively charged electrode; this form of energy was called a "cathode ray". In 1897, the British physicist J. J. Thomson (1856–1940) proved that atoms were not the most basic form of matter. He demonstrated that cathode rays could be deflected, or bent, by magnetic or electric fields, which indicated that cathode rays consist of charged particles.

More important, by measuring the extent of the deflection of the cathode rays in magnetic or electric fields of various strengths, Thomson was able to calculate the mass-to-charge ratio of the particles. These particles were emitted by the negatively charged cathode and repelled by the negative terminal of an electric field. Because like charges repel each other and opposite charges attract, Thomson concluded that the particles had a net negative charge; these particles are now called electrons. Most relevant to the field of chemistry, Thomson found that the mass-to-charge ratio of cathode rays is independent of the nature of the metal electrodes or the gas, which suggested that electrons were fundamental components of all atoms.

Aside: Electrostatic Forces

If two objects each have electric charge, then they exert an exert an electric force on each other. The magnitude of the force is linearly proportional the charge on each object and inversely proportional to square distance between each other. The magnitude of this electrostatic force is linearly proportional the distance between them and involves the existence of two types of charge, the observation that like charges repel, unlike charges attract and the decrease of force with distance.

The SI unit of electric charge is the coulomb (C) named after French physicist Charles-Augustin de Coulomb.

The video below shows JJ Thompson used such tube to measure the ratio of charge over mass of an electron.

Millikan’s Oil Drop Experiment: Measuring the Charge of the Electron

The American scientist Robert Millikan (1868–1953) carried out a series of experiments using electrically charged oil droplets, which allowed him to calculate the charge on a single electron. Millikan created microscopic oil droplets, which could be electrically charged by friction as they formed or by using X-rays. These droplets initially fell due to gravity, but their downward progress cou ld be slowed or even reversed by an electric field lower in the apparatus. By adjusting the electric field strength and making careful measurements and appropriate calculations, Millikan was able to determine the charge on individual drops (Figure \(\PageIndex{2}\)).

Looking at the charge data that Millikan gathered, you may have recognized that the charge of an oil droplet is always a multiple of a specific charge, \(1.6 \times 10^{−19}\, C\). Millikan concluded that this value must therefore be a fundamental charge—the charge of a single electron—with his measured charges due to an excess of one electron (\(1 \times (1.6 \times 10^{−19}\, C)\)), two electrons (\(2 \times (1.6 \times 10^{−19}\, C)\)), three electrons (\(3 \times (1.6 \times 10^{−19}\, C)\)), and so on, on a given oil droplet.

Defintion: Elementary Charge

The charge of an electron is sometimes referred to as the elementary charge and usually denoted by \(e\). The elementary charge is a fundamental physical constant and as of May 2019, its value is defined to be exactly \(1.602176634 \times 10^{−19}\, C\).

Since the charge of an electron was now known due to Millikan’s research, and the charge-to-mass ratio was already known due to Thomson’s research (\(1.759 \times 10^{11}\, C/kg\)), it only required a simple calculation to determine the mass of the electron as well.

\[\begin{align*} \mathrm{Mass\: of\: electron} &= \mathrm{1.602\times 10^{-19}\:\cancel{C}\times \dfrac{1\: kg}{1.759\times 10^{11}\:\cancel{C}}} \\[4pt] &= \mathrm{9.107\times 10^{-31}\:kg} \end{align*}\]

Scientists had now established that the atom was not indivisible as Dalton's theory had postulated , and due to the work of Thomson, Millikan, and others, the charge and mass of the negative, subatomic particles—the electrons—were known. However, the positively charged part of an atom was not yet well understood.

Exercise \(\PageIndex{1}\)

In a Millikan's oil drop experiment done in alternate universe, the measured charges on drops are found to be \(8 \times 10^{-19}\, C\), \(12 \times 10^{-19}\, C \) and \(20 \times 10^{-19}\, C\). What is the elementary charge in this universe?

Millikan experiment involves confirm that the charges of the drops were all small integer multiples of the elementary charge. The charges on the drop are found to be multiple of 4. Hence the small charge are \(4 \times 10^{-19} C\).

Contributors and Attributions

Paul Flowers (University of North Carolina - Pembroke), Klaus Theopold (University of Delaware) and Richard Langley (Stephen F. Austin State University) with contributing authors.  Textbook content produced by OpenStax College is licensed under a Creative Commons Attribution License 4.0 license. Download for free at http://cnx.org/contents/[email protected] ).

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Understanding Thomson's Cathode Ray Experiment and Its Impact

By Rupert Cole on 18 December 2013

Jj thomson's cathode-ray tube.

Rupert Cole celebrates JJ Thomson’s birthday with a look at one of the star objects in our  Collider exhibition.

https://www.youtube.com/watch?v=3oNQ_-iLgmA

Holding the delicate glass cathode-ray tube in my hands, once used by the great physicist JJ Thomson , was an incredible treat, and an experience I will never forget.

I had read lots about Thomson’s famous experiments on the electron – the first subatomic particle to be discovered – but to actually see and touch his apparatus myself, to notice the blackened glass and the tube’s minute features that are omitted in books, brought the object to life. History suddenly seemed tangible.

Using more than one cathode-ray tube in 1897 for his experiments, Thomson managed to identify a particle 1,000 times smaller than the then known smallest piece of matter: a hydrogen atom. Cambridge’s Cavendish Laboratory , where Thomson spent his scientific career, also has an original tube in its collection.

Each tube was custom-made by Thomson’s talented assistant, Ebenezer Everett, a self-taught glassblower. Everett made all of Thomson’s apparatus, and was responsible for operating it – in fact, he generally forbade Thomson from touching anything delicate on the grounds that he was “exceptionally helpless with his hands”.

The quality of Everett’s glassblowing was absolutely crucial for the experiments to work.

Cathode-rays are produced when an electric current is passed through a vacuum tube. Only when almost all the air has been removed to create a high vacuum – a state that would shatter ordinary glass vessels – can the rays travel the full length of the tube without bumping into air molecules.

Thomson was able to apply electric and magnetic fields to manipulate the rays, which eventually convinced the physics world that they were composed of tiny particles, electrons, opposed to waves in the now-rejected ether.

Find out more about Thomson and the story of the first subatomic particle here , or visit the Museum to see Thomson’s cathode-ray tube in the Collider  exhibition. If you’re interested in the details of how Thomson and Everett conducted their experiments visit the Cavendish Lab’s outreach page here .

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• Cathode Ray Experiment

What are Cathode Rays?

Cathode rays are a beam of negatively charged electrons traveling from the negative end of an electrode to the positive end within a vacuum, across a potential difference between the electrodes.

How Do the Cathode Rays Work?

The cathode is a negative electrode, Anode is the positive electrode. Since electrons are repelled by the negative electrode, the cathode is the source of cathode rays inside a vacuum environment. When a potential difference is applied, the electrons jump to an excited state and travel at high speeds to jump back-and-forth inside the vacuum glass chamber and when some cathode rays certain molecules of the cathode screen, they emit light energy. A wire is connected from anode to cathode to complete the electrical circuit.

Construction of a Cathode Ray Tube

Its Basic Components are: -

Electron Gun Assembly: - It is the source of the electron beams. The electron gun has a heater, cathode, pre-accelerating anode, focusing anode and accelerating anode.

Deflecting Plates: - They produce a uniform electrostatic field only in one direction, and accelerate particles in only one direction.

Screen: - The inner layer of the screen is coated with phosphorus, and produces fluorescence when cathode rays hit the screen by a process of phosphorus excitation.

Aquadag: - It is an aqueous solution of graphite used to collect the secondary emitted electrons which are required to keep the cathode ray in electrical equilibrium.

What is the Cathode Ray Tube Experiment?

In 1897, great physician J.J. Thompson, conducted his first cathode ray tube experiment to prove that rays emitted from an electron gun are inseparable from the latent charge. He built his cathode ray tube with a metal cylinder on the other end. The metal had two small diversions(slits), leading to an electrometer that could measure a small electric charge. From the first experiment, he discovered that the electrometers stopped measuring electric charge. From this, he deduced that the electric charge and the cathode rays must be combined and are the same entity.

Then he conducted a Second experiment, to prove the charge carried by the cathode rays was negative or positive. Now, he put a negatively charged metal plate on one side of the cathode rays to go past the anode, and a positively charged metal plate on the other side. Instead of an electrometer at one end of the Cathode Ray Tube, he used a fluorescent coated tube that would glow where the cathode ray hit it. When the charged metal plates were introduced he found that the cathode rays bent away from the negative plate and towards the positive plate. This proved that the cathode rays were negatively charged.

Then he performed the third experiment, to know the nature of the particles and reduce the mass of the particles as they had too small of a mass to be calculated directly. For the experiment, he used the cathode ray tube and with a high applied potential difference between the two electrodes, with the negatively charged cathode producing the cathode rays. He had already deduced that the particles were negatively charged. Firstly, he applied an electric field in the path between anode and cathode and measured the deflections from the straight path. Now he applied a magnetic field across the cathode ray tube by using an external magnetic field. The cathode ray is deflected by the magnetic field. Now he changed the direction of the external magnetic field and found that the beam of electrons is deflected in the opposite direction. From this experiment, he concluded that the electrostatic deflection is the same as the electromagnetic deflection for the cathode rays and he was able to calculate the charge to mass ratio of the electron.

After these three experiments, he deduced that inside the atom there consist of a subatomic particle, originally named ‘corpuscle’, then changed to ‘electron’ which is 1800 times lighter than the mass of hydrogen atom (Lightest atom).

Formula Used

The derivation of the formula used to calculate the charge to the mass ratio:

For Electric Field the force on a particle is

Force(F)=Charge(Q)*Electric field(E) ---<1>

For Magnetic Field the force on a particle moving with velocity is:

F=q*velocity(v)*Magnetic Field(B) ---<2>

From 1 and 2 we get,

V=E/B ----<3>

From the definition of Force,

Acceleration(a)= Force(f)/mass(m) ----<4>

Combining 1 and 4

a=q*E/m ----<5>

From Newton’s law Of motion, vertical displacement is:

Y= (1/2)*a*t*t ----<6>

From 5 and 6

q/m=(2*y*v*v)/x*x*E

Cathode Ray Tubes (CRT) 

The cathode ray tube (CRT) is a vacuum tube, in which electrons are discharged from the cathode and accelerated through a voltage, and thereby gains acceleration of some 600 km/s for every volt. These accelerated electrons collide into the gas inside the tube, thereby allowing it to glow. This enables us to see the path of the beam. Helmholtz coil, a device for producing a region of nearly uniform magnetic field, is also used to apply a quantifiable magnetic field by passing a current through them.

A magnetic field will cause a force to act on the electrons which are perpendicular to both the magnetic field and their direction of travel. Thus, a circular path will be followed by a charged particle in a magnetic field. The faster the speed of a charged particle in a magnetic field, the larger the circle traced out in a magnetic field. Contrarily, the larger the magnetic field needed for a given radius of curvature of the beam. The paths of the electrons are distorted by the magnet in CRT Tv when they are brought near the screen. The picture on the screen appears when the electrons accurately hit phosphors on the back of the screen. Because of this, different colors of light are emitted on the screen when the electrons are impacted. Hence, the electrons are forced to settle in the wrong place, thereby causing the distortion of the image and the psychedelic colors.

Postulates of J.J. Thomson’s Atomic Model

After the Cathode ray tube experiment, Thomson gave one of the first atomic models including the newly discovered particle. 

His model stated: -

An atom resembles a sphere of positive charge with a negative charge present inside the sphere.

The positive charge and the negative charge were equal in magnitude and thus the atom had no charge as a whole and is electrically neutral.

His model resembles a plum pudding or watermelon. It assumed that positive and negative charge inside an atom is randomly spread across the whole sphere like the red part of the watermelon (positive charge) and the black seeds (negative charge).

Practical Uses of Cathode Ray Tube Experiment

In ancient times, the cathode ray tubes were used in the beam where the electron was considered with no inertia but have higher frequencies and can be made visible for a short time.

Many scientists were trying to get the secrets of cathode rays, while others were in search of the practical uses or applications of cathode ray tube experiments. And the first search was ended and released in 1897 which was introduced as Karl Ferdinand Braun’s oscilloscope. It was used for producing luminescence on a chemical affected screen in which cathode rays were allowed to pass through the narrow aperture by focusing into the beams that looked like a dot. This dot was passed for scanning across the screen which was represented visually by the electrical pulse generator. 

Then during the first two to three decades of the twentieth century, inventors continued to search the uses of cathode ray tube technology. Then inspired by Braun's oscilloscope, A. A. Campbell advised that a cathode ray tube would be used for projecting video images on the screen. But, this technology of the time did not get matched with the vision of Campbell-Swinton. It was only until 1922, when Philo T. Farnsworth developed a magnet to get focused on the stream of electrons on the screen, for producing the image. Thus, the first kind of it, Farnsworth, was quickly backed up by Zworykin’s kinescope, known as the ancestor of modern TV sets.

Nowadays, most image viewer devices are made with the help of cathode ray tube technology including the guns of electrons which are used in huge areas of science as well as medical applications. One such use for cathode-ray tube research is the microscope invented by Ernst Ruska in 1928. The microscope based on electrons uses the stream of electrons to magnify the image as the electrons have a small wavelength which is used for magnifying the objects which are very small to get resolved by visible light. Just like Plucker and Crookes work, Ernst Ruska used a strong field of magnetic lines for getting it focused on the stream of electrons into an image.

Solved Example:  

Question: The charge of an electron e=1.602∗10−19 and its is mass m=9.11∗10−31. The stopping potential of an electron traveling in a cathode ray tube is V=5V. Find the velocity of an electron traveling (where charge of an electron e=1.602∗10−19 and mass m=9.11810−31).

Answer: Here we need to find the velocity of traveling electrons using the given stopping potential.

We know that eV=12mv2, the charge(e) and mass(m) of the electron is also given as,

e=1.602∗10−19 and m=9.11∗10−31

By substituting the values of e, m, V.(1.602∗10−19)(5)

=12(9.11∗10−31)(v2)v2

=(1.602∗10−19)(5)(2)9.11∗10−31v

=1.33∗106m/s             

FAQs on Cathode Ray Experiment

1. What is the procedure of the Cathode Ray Experiment?

The apparatus of Cathode Ray Experiment is arranged in such a way that the terminals have high voltage with the internal pressure, which is reduced by removing the air inside the CRT. Because of the high voltage in the terminal,  the partial air inside it is ionized and hence gas becomes the conductor. The electric current propagates as a closed-loop circuit. In order to recognize and measure the ray produced, a dipole is set up. The cathode rays will begin deflecting and repel from the dipole and move towards the anode because of the dipole. The phosphorescent substance is arranged in such a way that the rays strike the substance. And hence, it causes small sparks of light, which detects the stream of rays.

2. What are Cathode ray tubes?

Cathode ray tubes (CRT) are a presentation screen that produces pictures as a video signal. Cathode ray tubes (CRT) is a type of vacuum tube that displays pictures when electron beams from an electron gun hit a luminous surface. The CRT produces electron beams, accelerates them at high speed, and thereby deflecting them to take pictures on a phosphor screen. Electronic presentation gadgets being the most established and least expensive electronic presentation innovation, were initially made with CRTs. CRTs work at any aspect ratio, at any resolution, and geometry without the need to resize the picture. CRTs work on the principle of an optical and electromagnetic phenomenon, called cathodoluminescence.

3. What are the applications of Cathode ray tubes?

The following are the applications of Cathode ray tubes.

The main components of a cathode ray tube (CRT) includes A Vacuum tube holding an electron cannon and a screen lined with phosphors.

The technology of Cathode ray tubes is used by Televisions and computer monitors. Three electron cannons correlate to corresponding types of phosphors in color CRTs, one for each main color viz red, green, and blue.

Ancient computer terminals and black and white televisions are examples of monochromatic CRTs.

cathode ray tube (CRT) is also used in oscilloscopes, which are machines that display and analyze the waveform of electronic signals.

A cathode ray amusement device was the very first video game to be produced, which were used in old military radar screens.

4. What are the basic principles of the CRT?

There are three basic principles of the CRT as the following:

Electrons are released into a vacuum tube from very hot metal plates.

The released electrons are accelerated and their direction of movement is controlled by using either a magnetic field from a coil that is carrying an electric current or by a voltage between metal plates.

A high-velocity beam of electrons hits some materials such as zinc sulfide. The spot is created on the fluorescent screen, and it causes material, called a phosphor, to glow, giving a spot of light as wide as the beam.

5.  How to understand the concept of the Cathode Ray Experiment easily?

Students can understand the concept of the Cathode Ray Experiment easily with the help of a detailed explanation of the Cathode Ray Experiment provided on Vedantu. Vedantu has provided here a thorough explanation of the Cathode Ray Experiment along with Cathode Rays, How Do the Cathode Rays Work, Construction of a Cathode Ray Tube, Postulates of J.J. Thomson’s Atomic Model, and Practical Uses of Cathode Ray Tube Experiment along with examples. Students can learn the concepts of all the important topics of Science subject on Vedantu.

NCERT Study Material

.

suggested that they do. He advanced the idea that cathode rays are really streams of very small pieces of atoms. Three experiments led him to this.: of an 1895 experiment by Jean Perrin, Thomson built a ending in a pair of metal cylinders with a slit in them. These cylinders were in turn connected to an electrometer, a device for catching and measuring electrical charge. Perrin had found that cathode rays deposited an electric charge. Thomson wanted to see if, by bending the rays with a magnet, he could separate the charge from the rays. He found that when the rays entered the slit in the cylinders, the electrometer measured a large amount of negative charge. The electrometer did not register much electric charge if the rays were bent so they would not enter the slit. As Thomson saw it, the negative charge and the cathode rays must somehow be stuck together: you cannot separate the charge from the rays.
.		when physicists tried to bend cathode rays with an electric field. Now Thomson thought of a new approach. A charged particle will normally curve as it moves through an electric field, but not if it is surrounded by a conductor (a sheath of copper, for example). Thomson suspected that the traces of gas remaining in the tube were being turned into an electrical conductor by the cathode rays themselves. To test this idea, he took great pains to extract nearly all of the gas from a tube, and found that now the cathode rays did bend in an electric field after all.
	from these two experiments, "I can see no escape from the conclusion that [cathode rays] are charges of negative electricity carried by particles of matter." But, he continued, "What are these particles? are they atoms, or molecules, or matter in a still finer state of subdivision?"
.		sought to determine the basic properties of the particles. Although he couldn't measure directly the mass or the electric charge of such a particle, he could measure how much the rays were bent by a magnetic field, and how much energy they carried. From this data he could calculate the of the mass of a particle to its electric charge ( / ). He collected data using a variety of tubes and using different gases.
.		Just as Emil Wiechert had reported earlier that year, the mass-to-charge ratio for cathode rays turned out to be far smaller than that of a charged hydrogen atom--more than one thousand times smaller. Either the cathode rays carried an enormous charge (as compared with a charged atom), or else they were amazingly light relative to their charge. was settled by . Experimenting on how cathode rays penetrate gases, he showed that if cathode rays were particles they had to have a mass very much smaller than the mass of any atom. The proof was far from conclusive. But experiments by others in the next two years yielded an independent measurement of the value of the charge ( ) and confirmed this remarkable conclusion.
the hypothesis that "we have in the cathode rays matter in a new state, a state in which the subdivision of matter is carried very much further than in the ordinary gaseous state: a state in which all matter... is of one and the same kind; this matter being the substance from which all the chemical elements are built up."
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Cathode Ray Experiment , also known as the Crookes tube experiment , is a historically significant experiment in the field of physics that helped scientists understand the nature of electrons. English scientist Sir J.J. Thomson performed an experiment using a Cathode Ray Tube, which led to the discovery of an electron.

In this article, we will discuss this significant experiment, including details of the Cathode Ray Tube, the procedure of the experiment, and J.J. Thomson’s observations, which led to one of the greatest discoveries in the field of science.

Table of Content

• What is the Cathode Ray Experiment?

What is Cathode Ray Tube (CRT)?

• Experiment Setup

Applications of Cathode Ray Experiment

• Limitations of the Cathode Ray Experiment

What is Cathode Ray Experiment?

Cathode Ray Experiment, also known as the Cathode Ray Tube (CRT) Experiment, is a fundamental experiment in the history of physics that played a crucial role in understanding the nature of electrons and contributed to the development of modern electronics and television technology.

The experiment was first conducted by Sir William Crookes in the 1870s and later improved upon by scientists like J.J. Thomson in the late 19 th and early 20 th centuries.

Who is J.J. Thomson?

Joseph John Thomson, often called J.J. Thomson, was a British physicist celebrated for winning the Nobel Prize in Physics in 1906 for his research on how electricity moves through gases. His notable achievement was the discovery of the electron during the Cathode Ray Experiment.

A Cathode Ray Tube (CRT) is a special glass tube that played a big part in J.J. Thomson’s important experiment. This clever device helped scientists understand tiny particles that make up atoms.

Structure of CRT

CRT has a simple structure. It’s a sealed glass tube with two electrodes at each end – one is called the cathode (negative), and the other is the anode (positive). When these electrodes are connected to power, they create an electric field inside the tube. The tube is made empty, like a vacuum, so there’s no air inside.

The vacuum is essential because it lets cathode rays move in a straight line from the cathode to the anode without any interference from air. This controlled setup helps scientists study the behavior of cathode rays in different situations. The CRT is a key tool that led to important discoveries about the tiniest building blocks of matter.

Cathode Ray Experiment Setup

Below is the detailed setup for the Cathode Ray Tube Experiment with the elements used along with the diagram:

• Cathode Ray Tube (CRT): A sealed glass tube with a cathode and anode at either end.
• Cathode: A negatively charged electrode inside the CRT.
• Anode: A positively charged electrode inside the CRT.
• High Voltage Generator: A power supply capable of providing a high voltage between the cathode and anode.
• Vacuum Pump: A pump to evacuate air from the CRT to create a low-pressure environment.
• Discharge Tube: The entire CRT assembly including the cathode, anode, and vacuum space.
• Perforated Anode Disk: Placed at the anode end to allow some cathode rays to pass through.

Procedure of Experiment

Below is the procedure steps for the experiment with the perspective of the JJ Thomson:

• JJ Thomson created a sealed cathode ray tube with minimal air inside.
• Connected the tube to a power source, causing electrons (cathode rays) to shoot out.
• Observed electrons moving in straight lines inside the vacuum of the tube.
• Introduced an electric field by adjusting the power, causing electrons to change their path.
• Experimented with magnets, observing electrons being affected and swerving in response.
• Adjusted power settings to observe changes in electron movement, establishing consistent patterns.
• Systematically recorded electron behavior in various situations.
• Determined the charge-to-size ratio of electrons, making a significant discovery.
• Concluded that cathode rays were composed of tiny particles known as electrons.
• Thomson’s discovery revolutionized understanding of the microscopic world’s building blocks.

Observation of Cathode Ray Experiment

In the Cathode Ray Experiment, J.J. Thomson made a ground breaking observation i.e., when cathode rays encountered electric and magnetic fields, they exhibited intriguing behavior. Thomson noticed their deflection, and the direction of this deflection pointed to a negative charge. This pivotal observation led Thomson to the groundbreaking conclusion that cathode rays were composed of negatively charged particles, now recognized as electrons.

Conclusion of Cathode Ray Experiment

Cathode Ray Experiment marked a revolutionary moment in the realm of science. J.J. Thomson’s demonstration of cathode ray deflection and the identification of these rays as negatively charged particles conclusively affirmed the existence of subatomic particles. This groundbreaking experiment transformed our comprehension of atomic structure, shattering the notion that atoms were indivisible. Instead, Thomson’s work revealed the presence of smaller components within atoms. This pivotal episode in the history of physics not only altered fundamental perspectives but also laid the foundation for subsequent advancements in the field.

The Cathode Ray Experiment, conducted by Sir J.J. Thomson in 1897, led to several significant applications and advancements in various fields:

• Discovery of the Electron: The most direct outcome of the Cathode Ray Experiment was the discovery of the electron, a fundamental component of atoms. This discovery was pivotal in the development of atomic theory and quantum physics.
• Television and Computer Monitors: The technology behind cathode ray tubes (CRTs) was essential in the development of early television and computer monitors. These devices used electron beams, controlled and focused by magnetic or electric fields, to illuminate phosphors on the screen, creating images.
• Medical Imaging: Cathode ray technology found applications in medical imaging, particularly in early forms of X-ray machines and later in more advanced imaging technologies.
• Electron Microscopy: The principles discovered in the Cathode Ray Experiment were integral to the development of electron microscopy, which uses a beam of electrons to create an image of a specimen. This technology allows for much higher resolution than traditional light microscopy.

Limitations of Cathode Ray Experiment

The Cathode Ray Experiment, while groundbreaking in its time, had several limitations:

• Lack of Precise Measurement Tools: At the time of Thomson’s experiments, the precision and accuracy of measurement tools were limited. This meant that the measurements of the charge-to-mass ratio of electrons were not as accurate as what can be achieved with modern equipment.
• Incomplete Understanding of Subatomic Particles: Thomson’s experiment was conducted at a time when the structure of the atom was not fully understood. This meant that while the experiment led to the discovery of the electron, it did not provide a complete picture of subatomic particles and their interactions.
• Limited Control over Experimental Conditions: The vacuum technology and methods to control the electric and magnetic fields in Thomson’s time were rudimentary compared to today’s standards. This limited the ability to control experimental conditions precisely.
• Atomic Structure
• Discovery of Electrons

Cathode Ray Experiment – FAQs

J.J. Thomson, whose full name is Joseph John Thomson, was a British physicist born on December 18, 1856, in Cheetham Hill, Manchester, England, and he passed away on August 30, 1940. He is best known for his discovery of the electron, a fundamental subatomic particle.

What are Cathode Rays?

Cathode rays are streams of electrons observed in a vacuum when a high voltage is applied between electrodes in a cathode ray tube (CRT). These rays were first discovered and studied by J.J. Thomson in the late 19th century.

What was the Cathode Ray Experiment?

The cathode ray experiment, conducted by J.J. Thomson in the late 19th century, was a series of experiments that led to the discovery of electrons and provided crucial insights into the nature of subatomic particles.

What are Two Conclusions of the Cathode Ray Experiment?

Two conclusion of Cathode Ray Experiment are: Cathode rays are streams of negatively charged particles (electrons). These particles are fundamental components of all atoms.

Why did J.J. Thomson Experimented with Cathode?

J.J. Thomson experimented with cathode rays to investigate their nature and to understand the internal structure of atoms.

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Why did J.J. Thomson observe protons in his cathode ray experiment?

In this MIT lecture , at 7:22, the professor says that when J.J. Thomson added a positively charged plate on one side of the cathode ray and a negatively charged plate on the other side, he observed a large deflection towards the positive plate, and a small deflection towards the negative plate (see image below).

This observation implied that hydrogen gas consists of positively charged particles with lots of mass, and negatively charged particles with a small amount of mass.

However, according to Wikipedia , cathode rays are "streams of electrons observed in vacuum tubes." Thus, my question is: in J.J. Thomson's experiment, where are the protons (positively charged particles) coming from?

• atomic-structure

• 2 $\begingroup$ You might need to reverse the polarity of the high voltage source for that to happen. $\endgroup$ –  Zhe Commented Jan 23, 2019 at 22:20
• $\begingroup$ I have been wondering the same as well. There's an anode ray experiment by Goldstein to detect positive charge but the way the professor mentions Thompson's experiment (for the positive charge part) doesn't seem to be correct $\endgroup$ –  Jon Commented Mar 23, 2022 at 7:38

2 Answers 2

Thompson observed more than just cathode rays.

He experimented with different gases and different pressures in his tubes. When there is a significant amount of hydrogen you get ionisation of the gas and more than one type of charged particle can be generated and deflected (though different conditions are needed to see protons than electrons).

Part of the point of this was to distinguish between particles produced by the gas and particles produced when there was little or no gas. The point about cathode rays is that they can still be produced when there is virtually no gas in the tube by thermionic emission from the cathode. They have the same properties as one of the particles produced by ionising hydrogen which established some of the basis for saying that electrons are an important part of atoms.

In the diagram both the ends of battery group is shown as cathode(small line)(?)

Here a perforated anode is used and so only anions-here electrons- originated from cathode can pass through and so beyond anode only stream of electrons would emerge which registers a high deflection towards positive plate in the external electric field, due to their very small mass and so low momentum.

If the applied high DC voltage is reversed in the discharge tube( not of the external electric field), a stream of protons (hydrogen atom minus electron) would emerge out of perforation in the cathode (they originated from anode) would pass though and in the external field of the same strength as in the earlier experiment, would register less deflection due to heaviness of proton(nearly 1836 times that of electron) and so high momentum.

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