- 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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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
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Cathode Ray Tube (CRT)
Cathode ray tube definition.
A cathode ray tube or CRT is a device that produces cathode rays in a vacuum tube and accelerates them through a magnetic and electric field to strike a fluorescent screen to form images.
Cathode Ray Tube History
The eminent physicist Johann Hittorf discovered cathode rays in 1869 in Crookes tubes. Crookes tubes are partially vacuum tubes having two electrodes kept at a high potential difference to discharge cathode rays from the negatively charged electrode cathode. Arthur Schuster and William Crooks proved that cathode rays are deflected by electric and magnetic fields, respectively. In the year 1897, the English physicist J.J. Thomson’s experiments with cathode rays led to the discovery of the electron , the first subatomic particle to be discovered.
The earliest version of the cathode ray tube, Braun Tube, was invented in 1897 by the German physicist Ferdinand Braun. It employed a cold cathode for working. He used a phosphor-coated mica screen and a diaphragm to produce a visible dot. The cathode beam was deflected by a magnetic field only, in contrast to the discharge tube used earlier in the same year by J.J. Thomson, which employed only electrostatic deflection using two internal plates. Braun is also credited with the invention of the cathode ray tube oscilloscope, also known as Braun’s Electrometer.
In 1907, the cathode ray tube was first used in television when Russian scientist Boris Rosing passed a video signal through it to obtain geometric shapes on the screen. Earlier cathode ray tubes used cold cathodes. However, a hot cathode came into existence after being developed by John B. Johnson and Harry Weiner Weinhart of Western Electric. This type of cathode consists of a thin filament heated to a very high temperature by passing an electric current through it. It uses thermionic emissions in vacuum tubes to release electrons from a target.
The first commercial cathode ray tube television manufacture dates back to 1934 by the company Telefunken in Germany. This curved the path for large-scale manufacture and use of CRT TVs until the recent development of Liquid Crystal Displays, Light Emitting diodes, and Plasma TVs.
Cathode Ray Tube Description
The CRT is composed of three parts.
Electron Gun
This part produces a stream of electrons traveling at very high speeds by the process of thermionic emission. A thin filament is heated up by the passage of alternating current through it. It is used to heat the cathode, generally made of the metal cesium, which releases a stream of electrons when heated to temperatures of about 1750 F. The anode, which is the positively charged electrode, is placed a small distance away and is maintained at a high voltage which forces the cathode rays to gain considerably high accelerations as they move towards it.
The stream of electrons passes through a small aperture in the anode to land in the central part of the tube. There is a grid or a series of grids maintained at a variable potential, which control(s) the intensity of the electron beam reaching the anode. The brightness of the final image formed on the screen is also restricted thus. A monochrome CRT has a single electron gun, whereas a color CRT has three electron guns for the primary colors, red, green, and blue, which overlap among themselves to produce colored images.
Deflection System
The electron stream, after coming out of the anode, tends to spread out in the form of a cone. But it needs to be focused to form a sharp point on the screen. Also, its position on the screen should be as desired. This is achieved by subjecting the beam to magnetic and electric fields perpendicular to each other. The straight path of the beam then gets deflected, and it hits the screen at the desired point. It should be kept in mind that the anode gives it a considerable acceleration of the order of fractions of the speed of light. This endows the beam with very high amounts of energy.
Fluorescent CRT Screen
This part projects the image for the user’s view. It is given a coating of zinc sulfide or phosphorus which can produce fluorescence. When the highly energetic beam of electrons strikes it, its kinetic energy is converted to light energy, thus forming an illuminated spot on the screen. When complex signals are applied to the deflection system, the bright spot races across the screen horizontally and vertically, forming what is called the raster.
The raster scanning takes place in the same way as we would read a book. That is, from left to right, then go down and back to the left and move right to finish reading the line. This continues until the full screen is finished scanning. However, the CRT scan takes place so rapidly every second that the viewer cannot follow the actual movement of the dot but can see the whole image so produced.
Cathode Ray Tube Mechanism Video
Cathode ray tube experiment by j.j.thomson.
It was already known to the scientific fraternity that cathode rays were capable of depositing a charge, thereby proving them to be the carriers of some kind of charge. But they were not really sure whether this charge could be separated from the particles forming the rays. Hence, the celebrated English physicist J. J. Thomson devised an experiment to test the exact nature.
Thomson’s First CRT Experiment
Thomson took a cathode ray tube, and at the place where the electron beam was supposed to strike, he positioned a pair of metal cylinders having slits on them. The pair, in turn, was connected to an electrometer, a device for catching and measuring electric charges. Then, on operating the CRT, in the absence of any electric or magnetic fields, the beam of electrons traveled straight up to the cylinders, passed through the aptly positioned slits, and made the electrometer register a high amount of charge. So far, the result was quite an expected one.
In the next step, he put a magnet in the vicinity of the cathode ray path that set up a magnetic field. Now, as you may know, an electric field and a magnetic field can never act along the same line. Hence, the charged cathode rays get deflected from their path and give the slits a miss. The electrometer, hence, fails to register anything whatsoever. Thus, he concluded the cathode rays carry the charges along with them wherever they go, and it is impossible to separate the charges from the rays.
Thomson’s Second CRT Experiment
In his second attempt, Thomson tried to deflect the cathode rays by applying an electric field. It could prove the nature of the charge carried by them. There had been attempts before to achieve the end, but they had failed. He thought that if the streams are electrically charged, then they should be deflected by electric fields, but he could not explain why his setup failed to show any such movement.
He later came up with the idea that there was no change from the original path as the stream was covered by a conductor, that is, a layer of ionized air in this case. So he took great pains to make the interior of the tube as close to a vacuum as he could by drawing out all the residual air, and bravo! There was a pronounced deflection in the cathode rays. The great scientist had cleverly put two electrodes, positive and negative, halfway down the tube to produce the electric field. On observing that the beam deflected towards the anode, he could successfully prove that the cathode rays carried one and only one type of charge, negative.
Thomson’s Third CRT Experiment
Thomson tried to calculate the charge-to-mass ratio of the particles constituting the rays and found it to be exceptionally small. That implies the particles have either a very small mass or a very high charge. He decided on the former and gave a bold hypothesis that cathode rays were formed of particles emanating from the atom itself.
Experiment Summary
By using certain modifications in the regular CRT, Thomson’s cathode ray tube experiment proved that cathode rays consist of streams of negatively charged particles having smaller masses than that atoms. It was highly likely for them to be one of the components of atoms.
Cathode Ray Tube Applications
Oscilloscope.
It measures the changes in electrical voltage with time. If the horizontal plate is attached to a voltage source and the vertical to a clocking mechanism, then the variations in the magnitude of the voltage will show up on the CRT monitor in the form of a wave. With an increase in voltage, the line forming the wave shoots up while it comes down if the voltage is low. If, instead of a variable voltage source, the horizontal plates are connected to a circuit, then the arrangement can be used to detect any sudden change in its voltage. Thus, it can be used for troubleshooting purposes.
Televisions
Before the emergence of lightweight LCD and plasma TVs, all televisions were bulky and had cathode ray tubes in them. They had a very fast raster scan rate of about 1/50 th of a second. In a color TV, the persistence of the different colors would last for only the time between two consecutive scans. If it stayed longer, then the tube would produce blurred images. But if the effect of the colors ended before the next scan, then it gave rise to a flickering screen. Modern tube TVs use flat-screen CRTs, unlike their yesteryear counterparts.
Cathode Ray Tube Amusement Device
The predecessor to modern video games, the cathode ray tube amusement device gave the world the first gaming device. The CRT produced electronic signals in the form of a ray of light. Controller knobs in the tube were then used to adjust the trajectories of light so that it could hit on a target imprinted on a clear overlay attached to the CRT display screen. The game was conceptualized on World War II missile displays and created the effect of firing missiles at targets.
Other Applications
Cathode ray tube monitors are widely used as display devices in radars. However, the CRT computer monitor has gradually become obsolete with the introduction of TFT-LCD thin panel monitors.
Health Risks
Ionizing Radiation : CRTs can emit a small amount of ionizing radiation that needs to be kept under control by the Food and Drug Administration Regulations in 21 C.F.R. 1020.10. However, most CRTs manufactured after 2007 have much lesser emissions than the prescribed limit.
Flicker: Low refresh rates, 60Hz and below, can produce flicker in most people, although the susceptibility of eyesight to flicker varies from person to person.
Toxicity: Modern-day CRTs may have their rear glass tubes made of leaded glass, which is difficult to dispose of as they can cause an environmental hazard. Some of the older versions also contain cadmium and phosphorus, making the tubes highly toxic. Special cathode ray tube recycling processes fulfilling the norms of the United States Environmental Protection Agency should be followed.
Implosion: Very high levels of vacuum inside a CRT can cause it to implode if there is any damage to the covering glass. This is caused by the high atmospheric pressure, which forces the glass to crack and fly off at high speeds in all directions. Though modern CRTs have strong envelopes to prevent shattering, they should be handled very carefully.
Noise: The signal frequencies used to operate CRTs are of a very high range and are usually imperceptible to the human ear. However, small children can sometimes hear very high-pitched noises near CRT televisions. That is because they have a greater sensitivity to hearing.
The cathode ray tube was a useful invention in Science for the discovery of an important fundamental particle like an electron and also opened up newer arenas of research in atomic Physics. Until about the year 2000, it was the mainstay of televisions all over the world before being forced into oblivion due to the emergence of newer technologies.
https://en.wikipedia.org/wiki/Cathode_ray
https://www.chemteam.info/AtomicStructure/Disc-of-Electron-History.html
https://www.techtarget.com/whatis/definition/cathode-ray-tube-CRT
https://explorable.com/cathode-ray-experiment
http://www.scienceclarified.com/Ca-Ch/Cathode-Ray-Tube.html
Article was last reviewed on Tuesday, May 9, 2023
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One response to “Cathode Ray Tube (CRT)”
I want to ask that in cathode ray tube tv why electrons are never finish which is on cathode while the material have limited electrons
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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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- 1 Physics Experiments
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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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Martyn Shuttleworth (Sep 22, 2008). Cathode Ray Experiment. Retrieved Sep 03, 2024 from Explorable.com: https://explorable.com/cathode-ray-experiment
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The cathode ray tube was a scientific curiosity discovered in the late 19 th century, and a mainstay of display technology in the late 20 th . We now know that the mysterious ‘cathode rays’ are in fact electrons—and we can use magnets to bend their path.
This experiment obviously requires a cathode ray tube filled with gas which glows when electrons hit it. The ideal CRT is enclosed by Helmholtz coils to allow a varying magnetic field to be applied. In the absence of Helmholtz coils, a strong neodymium magnet should suffice to bend the electron beam.
In addition to a cathode ray tube, you’ll probably need a sensitive camera to show your audience the results of this experiment. The beams of electrons are too dim for anything except a very small audience to see directly, and are something of a challenge for video equipment too! A camera with a night mode, or manual control over gain (or ISO) and shutter speed will probably be necessary.
If you’ve not got a cathode ray tube, an old CRT TV or computer monitor and a strong magnet will provide a more qualitative version of this demo.
The demonstrations
- Dim the lights and turn on the camera if you’re using one.
- Turn up the energy of the electron beam until the gas inside the globe is clearly glowing.
- If your CRT doesn’t have Helmholtz coils, simply wave the neodymium magnet near the CRT to show the beam bending. You may need to do this quite slowly if the camera is set to a low frame rate to increase its low light sensitivity.
- If your CRT does have Helmholtz coils, turn up the current in them until the beam bends.
- Having curved the path of the beam, turn up the energy further and show that the curvature decreases with increasing electron energy.
- Apply a higher magnetic field to demonstrate that the curvature can again be increased by increasing the magnetic field strength.
CRT TV/monitor + magnet
- Get an image on the television or computer screen. If it’s a computer screen simply plugging it into a laptop should work. For a TV, many camcorders and digital stills cameras will have an S-video, component or composite connection; older camcorders may have these directly, but newer camcorders or digital cameras may have a bespoke cable which plugs into a mini-USB or similar jack on the camera and feeds out to multiple types of connector for insertion into the TV. A relatively still, bright image or video makes the effect we’re about to observe easier to distinguish.
- Put the strong magnet near to the TV screen. The image will warp, and sweeping trails of colour will appear.
- If the distortion and colours remain after taking the magnet away from the TV, turning it off and on again should force the TV to ‘degauss’ which will fix the problem—this is signified by the distinctive clunk which often accompanies a CRT turning on. Sometimes, often after repeated cycling, the TV will fail to degauss. In this case, turn it off, leave it for a short period, and turn it on again.
Vital statistics
speed of an electron accelerated through 1 V: 600 km/s
strength of the LHC bending magnets: 8.36 T
How it works
The key here is that magnetic fields will bend the path of a moving charged particle, and we can make use of this effect to control a beam. Crucially for the Accelerate! recipe, you need a larger magnetic field to bend a faster-moving particle.
In the cathode ray tube, electrons are ejected from the cathode and accelerated through a voltage, gaining some 600 km/s for every volt they are accelerated through. Some of these fast-moving electrons crash into the gas inside the tube, causing it to glow, which allows us to see the path of the beam. Helmholtz coils can then be used to apply a quantifiable magnetic field by passing a known current through them.
A magnetic field will cause a force to act on the electrons which is perpendicular to both their direction of travel and the magnetic field. This causes a charged particle in a magnetic field to follow a circular path. The faster the motion of the particle, the larger the circle traced out for a given field or, conversely, the larger the field needed for a given radius of curvature of the beam. Making this quantitative point is impossible without control over both particle energy and magnetic field, so this will need to be stated if your demo doesn’t have both of these.
In the case of the CRT TV, the paths of the electrons are distorted by the magnet being brought near the screen. The picture on the screen is dependent on the electrons precisely hitting phosphors on the back of the screen, which emit different colours of light when impacted. The electrons are thus forced to land in the wrong place, causing the distortion of the image and the psychedelic colours.
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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.
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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Cathode ray tube experiments.
April 18, 2017 English Posts , Quantum Physics 53,735 Views
A Crookes tube is an early experimental electrical discharge tube, with vacuum, invented by English physicist William Crookes and others around 1869-1875, in which cathode rays , streams of electrons , were discovered.
Developed from the earlier Geissler tube, the Crookes tube consists of a partially evacuated glass bulb of various shapes, with two metal electrodes, the cathode and the anode , one at either end. When a high voltage is applied between the electrodes, cathode rays (electrons) are projected in straight lines from the cathode. It was used by Crookes, Johann Hittorf, Julius Plücker, Eugen Goldstein, Heinrich Hertz, Philipp Lenard and others to discover the properties of cathode rays, culminating in J.J. Thomson’s 1897 identification of cathode rays as negatively charged particles, which were later named electrons . Crookes tubes are now used only for demonstrating cathode rays.
Wilhelm Röntgen discovered X-rays using the Crookes tube in 1895. The term Crookes tube is also used for the first generation, cold cathode X-ray tubes, which evolved from the experimental Crookes tubes and were used until about 1920.
The picture below illustrates the operation of a Crookes tube in a schematic way.
For experiments with cathode ray tube we used an educational model readily available on eBay. It is a tube equipped with cathode and anode, with a mask with a slit so as to produce a planar cathode ray beam, there is also a fluorescent screen so as to highlight in a visual manner the presence of the beam. Two plates, used to demonstrate the influence of an electric field on the electron beam, complete the fixture. The images below show the apparatus.
The high voltage between anode and cathode is obtained with a HV generator powered with a 12 V, it generates a voltage value of 10 KV. The high voltage generator is a compact module easily available online, however you can use any HV generator, such as the ones based on diodes and capacitors in Cockcroft-Walton voltage multiplier configuration.
Magnetic Deflection
As we know, cathode rays are made up of electrons that are accelerated from the cathode to the anode. These are therefore negatively charged moving particles in a straight line, as such they are subject to the Lorentz force , which governs the motion of charged particles within magnetic fields. Generally speaking, the force exerted on a charge q moving with speed v within a magnetic field B and electric field E , is calculated with the following formula :
In our case, the electric field is null and the moving charged particles are electrons, so the formula becomes:
F = – e (v x B)
Given the vector product between v and B , the force is perpendicular to both the direction of the magnetic field and the velocity direction. The picture below illustrates the effect of Lorentz interaction.
This phenomena can be easily verified by using our cathode tube and a permanent neodymium magnet: by approaching the magnet to the tube, the beam is deflected low or upward depending on the polarity of the magnet. The image and video below show this experimental test.
Electric Deflection
The cathode ray beam is obviously sensitive to electric fields as well. From the law of Lorentz we get:
F = – e E
The force is proportional to the intensity of the electric field, and is directed in the opposite direction since the charge of the electron is negative, as shown in the picture below.
This experimental test can also be easily carried out with our cathode tube, as it is provided with a pair of deflection plates positioned close to the beam. By applying a voltage between 500 and 800 V, a clear deflection of the beam may be obtained. The image and video below show this experimental test.
https://youtu.be//QpcbeExV1LA?rel=0
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Tags Cathode Ray Tube
Gamma Spectroscopy with KC761B
Abstract: in this article, we continue the presentation of the new KC761B device. In the previous post, we described the apparatus in general terms. Now we mainly focus on the gamma spectrometer functionality.
After Dalton's breakthrough of atomic theory, scientists tried to determine the masses of atoms from the fraction of elements in compounds. Since an individual atom is so small, the mass of the atoms of one of the element was determined relative to the mass of the atoms of another element, based on a mass standard. Dalton's model is very important because it was based on the observation of many chemical reactions and the masses of reacting elements which could be explained in terms of atoms. But for the explanation of drawbacks that are discussed need a more complex atomic model. The discovery of fundamental (subatomic) particles leads to the nuclear model of an atom.
Discovery of Electrons Cathode rays are a type of radiation emitted by the negative terminal, the cathode, and was discovered by passing electricity through glass tubes from which the air was mostly evacuated. One device used to investigate this phenomenon was a cathode ray tube, the forerunner of the television tube. It is a glass tube from which most of the air has been evacuated. When the two metal plates are connected to a high–voltage source, the negatively charged plate, called the cathode, emits an invisible ray. The cathode ray is drawn to the positively charged plate, called the anode, where it passes through a hole and continues traveling to the other end of the tube. When the ray strikes the specially coated surface, it produces a strong fluorescence, or bright light.
In some experiments, two electrically charged plates and a magnet were added to the outside of the cathode ray tube. In the presence of magnetic field, the cathode ray strikes point ‘B’ whereas in the presence of electric field, the cathode ray strikes point ‘A’ as shown in the figure. In the presence or absence of both electric and magnetic fields such that their magnitudes cancel out each other, the cathode ray strikes point 'C'. According to electromagnetic theory, a moving charged body behaves like a magnet and can interact with electric and magnetic field through which it passes. Because the cathode ray is attracted by the plate bearing positive charges and repelled by the plate bearing negative charges, it must consist of negatively charged particles. These negatively charged particles are called as electrons.
- Cathode rays travel in straight lines. That is why, cathode rays cast shadow of any solid object placed in their path. The path cathode rays travel is not affected by the position of the anode.
- Cathode rays consist of matter particles, and posses energy by the virtue of its mass and velocity. Cathode rays set a paddle wheel into motion when it is placed in the path of these rays one the bladder of the paddle wheel.
Cathode rays consist of negatively charged particles. When cathode rays are subjected to an electrical field, these get deflected towards the positively charge plate(Anode).
We know that a positively charged body would attract only a negatively charged body, therefore the particles of cathode rays carry negative charge.
Cathode rays also get deflected when these are subjected to a strong magnetic field.
- Cathode rays heat the object only which they fall. The cathode ray particles possess kinetic energy. When these particles strike an object, a part of the kinetic energy is transferred to the object this causes a rise in the temperature of the object.
- Cathode rays cause green fluorescence on glass surface, i.e., the glass surface only which the cathode rays strike show a colored shine.
- Cathode rays can penetrate through thin metallic sheets.
- Cathode rays ionize the gases through which they travel.
- Cathode rays travel with speed nearly equal to that of light.
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IMAGES
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COMMENTS
Learn how J.J. Thomson discovered electrons by using a cathode ray tube, a vacuum tube that produces a beam of electrons. Find out the apparatus setup, the procedure of the experiment, the conclusion and the uses of cathode ray tubes.
Learn how JJ Thomson discovered the electron using a cathode ray tube in 1897. Find out the importance of this discovery for physics and chemistry, and some facts about Thomson and his work.
A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. [2] The images may represent electrical waveforms on an oscilloscope , a frame of video on an analog television set (TV), digital raster graphics on a computer monitor , or ...
To see all my Chemistry videos, check outhttp://socratic.org/chemistryJ.J. Thompson discovered the electron, the first of the subatomic particles, using the ...
Learn about the history, definition, and mechanism of cathode ray tubes, devices that produce and deflect electron beams to form images. See how J.J. Thomson used CRT experiments to discover the electron and its charge.
Learn how Thomson used a cathode ray tube to discover and study the nature of electrons, the negatively charged particles emitted from atoms. Explore his three experiments, his conclusions and their impact on modern physics.
Learn how to demonstrate the bending of electrons in a cathode ray tube using magnets or Helmholtz coils. Find out how this effect is used in CRT TVs and monitors, and how it relates to the LHC.
Learn about cathode rays, streams of electrons observed in discharge tubes, and their discovery, composition, and applications. Find out how cathode rays are deflected by electric or magnetic fields, and how they are used in vacuum tubes and cathode-ray tubes.
Learn how J.J. Thomson discovered the electron using a cathode ray tube and how his experiment changed the atomic theory. Explore key questions, explanations, and examples related to the cathode ray tube experiment.
In 1897, JJ Thomson discovered the electron in his famous cathode ray tube experiment. How did it work and why did Thomson do the experiment in the first pl...
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.
This chemistry and physics video tutorial provides a basic introduction into the cathode ray tube experiment. JJ Thompson used this experiment to conclude t...
Yes, J.J. Thomson used cathode rays. He used them in his experiments with cathode ray tubes in order to determine what cathode rays were composed of. He found that they were composed of a ...
J.J. Thompson discovered the electron, the first of the subatomic particles, using the cathode ray tube experiment. He found that many different metals release cathode rays, and that cathode rays were made of electrons, very small negatively charged particles. This disproved John Dalton's theory of the atom, and Thompson came up with the plum pudding model of the atom.
For experiments with cathode ray tube we used an educational model readily available on eBay. It is a tube equipped with cathode and anode, with a mask with a slit so as to produce a planar cathode ray beam, there is also a fluorescent screen so as to highlight in a visual manner the presence of the beam. Two plates, used to demonstrate the ...
The cathode ray is drawn to the positively charged plate, called the anode, where it passes through a hole and continues traveling to the other end of the tube. When the ray strikes the specially coated surface, it produces a strong fluorescence, or bright light. In some experiments, two electrically charged plates and a magnet were added to ...
If you're seeing this message, it means we're having trouble loading external resources on our website. If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.
In the mid 1800's scientists successfully passed an electric current through a vacuum in a glass tube. They saw a glow from the tube that seemed to emanate f...
The cathode ray tube was used in the experiments of Röntgen and J. J. Thomson that led to the discoveries of X-rays and the electron, respectively. Cathode ray tubes remain familiar objects today as a result of the popularity of the neon sign. In electrical engineering, the term "cathode ray tube" (abbreviated CRT in this context) is also used ...
Cathode Ray History. Electron Beams Lead to Discovery of Subatomic Particles. A cathode ray is a beam of electrons in a vacuum tube traveling from the negatively charged electrode (cathode) at one end to the positively charged electrode ( anode) at the other, across a voltage difference between the electrodes. They are also called electron beams.
Cathode Ray Experiment. 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.