millikan oil drop experiment wikipedia

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Robert Millikan

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• The Nobel Prize - Biography of Robert A. Millikan
• American Institute of Physics - Biography of Robert Andrews Millikan
• Robert Andrews Millikan - Student Encyclopedia (Ages 11 and up)

Robert Millikan (born March 22, 1868, Morrison, Illinois , U.S.—died December 19, 1953, San Marino , California) was an American physicist honored with the Nobel Prize for Physics in 1923 for his study of the elementary electronic charge and the photoelectric effect .

Millikan graduated from Oberlin College (Oberlin, Ohio) in 1891 and obtained a doctorate at Columbia University in 1895. In 1896 he became an assistant at the University of Chicago , where he became a full professor in 1910. During his time in Chicago as an assistant professor, he wrote for high-school and college students several physics textbooks that entered widespread use.

In 1909 Millikan began a series of experiments to determine the electric charge carried by a single electron . He began by measuring the course of charged water droplets in an electric field . The results suggested that the charge on the droplets is a multiple of the elementary electric charge, but the experiment was not accurate enough to be convincing. He obtained more precise results in 1910 with his famous oil-drop experiment in which he replaced water (which tended to evaporate too quickly) with oil . Millikan varied the electric voltage between two metal plates as an oil drop fell between them until the drop stopped falling. When the drop was stationary , the downward force of gravity on the drop equaled the upward electrical force on the charges in the drop, and then Millikan could measure how much charge the drop had.

In 1916 he took up with similar skill the experimental verification of the equation introduced by Albert Einstein in 1905 to describe the photoelectric effect , in which electrons are ejected from a metal plate when light falls on it. The photoelectric effect had puzzled physicists, but Einstein described the energy of the ejected electron as equal to h f - φ, where h is Planck’s constant, f is the frequency of the light, and φ is a property of the metal called the work function. Einstein’s description of the photoelectric effect as a quantum phenomenon was controversial, but Millikan’s measurements proved Einstein’s theory and obtained an accurate value of Planck’s constant . When the United States entered World War I in 1917, he became vice chairman of the National Research Council in Washington, D.C., where he helped scientists apply their research to the war effort. He returned to Chicago in 1919.

In 1921 Millikan left the University of Chicago to become director of the Norman Bridge Laboratory of Physics at the California Institute of Technology (Caltech) in Pasadena . There he undertook a major study of the radiation that the physicist Victor Hess had detected coming from outer space. Millikan proved that this radiation is indeed of extraterrestrial origin, and he named it “ cosmic rays .” As chairman of the executive council of Caltech from 1921 until his retirement in 1945, Millikan turned that school into one of the leading research institutions in the United States.

The Millikan Oil Drop Experiment

Theresa Knott / Wikimedia Commons / CC BY-SA 3.0

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Robert Millikan's oil drop experiment measured the charge of the electron . The experiment was performed by spraying a mist of oil droplets into a chamber above the metal plates. The choice of oil was important because most oils would evaporate under the heat of the light source, causing the drop to change mass throughout the experiment. Oil for vacuum applications was a good choice because it had a very low vapor pressure. Oil droplets could become electrically charged through friction as they were sprayed through the nozzle or they could be charged by exposing them to ionizing radiation . Charged droplets would enter the space between the parallel plates. Controlling the electric potential across the plates would cause the droplets to rise or fall.

Calculations for the Experiment

F d = 6πrηv 1

where r is the drop radius, η is the viscosity of air and v 1 is the terminal velocity of the drop.

The weight W of the oil drop is the volume V multiplied by the density ρ and the acceleration due to gravity g.

The apparent weight of the drop in air is the true weight minus the upthrust (equal to the weight of air displaced by the oil drop). If the drop is assumed to be perfectly spherical then the apparent weight can be calculated:

W = 4/3 πr 3 g (ρ - ρ air )

The drop is not accelerating at terminal velocity so the total force acting on it must be zero such that F = W. Under this condition:

r 2 = 9ηv 1 / 2g(ρ - ρ air )

r is calculated so W can be solved. When the voltage is turned on the electric force on the drop is:

F E = qE

where q is the charge on the oil drop and E is the electric potential across the plates. For parallel plates:

E = V/d

where V is the voltage and d is the distance between the plates.

The charge on the drop is determined by increasing the voltage slightly so that the oil drop rises with velocity v 2 :

qE - W = 6πrηv 2

qE - W = Wv 2 /v 1

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August, 1913: Robert Millikan Reports His Oil Drop Results

Robert Millikan’s famous oil drop experiment , reported in August 1913, elegantly measured the fundamental unit of electric charge. The experiment, a great improvement over previous attempts to measure the charge of an electron, has been called one of the most beautiful in physics history, but is also the source of allegations of scientific misconduct on Millikan’s part.

Robert Millikan was born in 1868 and grew up in rural Iowa, the second son of a minister. Millikan attended Oberlin College, earned his PhD from Columbia University, and then spent a year in Germany before taking a position at the University of Chicago.

By about 1906, Millikan had become a successful educator and textbook writer, but he knew that he hadn’t done any research of real scientific significance, and was eager to make his mark as a researcher.

J.J. Thomson had discovered the electron in 1897 and had measured its charge-to-mass ratio. The next step was to determine the electron’s charge separately. Thomson and others tried to measure the fundamental electric charge using clouds of charged water droplets by observing how fast they fell under the influence of gravity and an electric field. The method did give a crude estimate of the electron’s charge.

Millikan saw this opportunity to make a significant contribution by improving upon these measurements. He realized that trying to determine the charge on individual droplets might work better than measuring charge on whole clouds of water. In 1909 he began the experiments, but soon found that droplets of water evaporated too quickly for accurate measurement. He asked his graduate student, Harvey Fletcher, to figure out how to do the experiment using some substance that evaporated more slowly.

Fletcher quickly found that he could use droplets of oil, produced with a simple perfume atomizer. The oil droplets are injected into an air-filled chamber and pick up charge from the ionized air. The drops then fall or rise under the combined influence of gravity, viscosity of the air, and an electric field, which the experimenter can adjust. The experimenter could watch the drops through a specially designed telescope, and time how fast a drop falls or rises. After repeatedly timing the rise and fall of a drop, Millikan could calculate the charge on the drop.

In 1910 Millikan published the first results from these experiments, which clearly showed that charges on the drops were all integer multiples of a fundamental unit of charge. But after the publication of those results, Viennese physicist Felix Ehrenhaft claimed to have conducted a similar experiment, measuring a much smaller value for the elementary charge. Ehrenhaft claimed this supported the idea of the existence of “subelectrons.”

Ehrenhaft’s challenge prompted Millikan to improve on his experiment and collect more data to prove he was right. He published the new, more accurate results in August 1913 in the Physical Review . He stated that the new results had only a 0.2% uncertainty, a great improvement of over his previous results. Millikan’s reported value for the elementary charge, 1.592 x 10 -19 coulombs, is slightly lower than the currently accepted value of 1.602 x 10 -19 C, probably because Millikan used an incorrect value for the viscosity of air.

It appeared that it was a beautiful experiment that had determined quite precisely the fundamental unit of electric charge, and clearly and convincingly established that “subelectrons” did not exist. Millikan won the 1923 Nobel Prize for the work, as well as for his determination of the value of Plank’s constant in 1916.

But later inspection of Millikan’s lab notebooks by historians and scientists has revealed that between February and April 1912, he took data on many more oil drops than he reported in the paper. This is troubling, since the August 1913 paper explicitly states at one point, “It is to be remarked, too, that this is not a selected group of drops, but represents all the drops experimented upon during 60 consecutive days.” However, at another point in the paper he writes that the 58 drops reported are those “upon which a complete series of observations were made.” Furthermore, the margins of his notebook contain notes such as, “beauty publish” or “something wrong.”

Did Millikan deliberately disregard data that didn’t fit the results he wanted? Perhaps because he was under pressure from a rival and eager to make his mark as a scientist, Millikan misrepresented his data. Some have called this a clear case of scientific fraud. However, other scientists and historians have looked closely at his notebooks, and concluded that Millikan was striving for accuracy by reporting only his most reliable data, not trying to deliberately mislead others. For instance, he rejected drops that were too big, and thus fell too quickly to be measured accurately with his equipment, or too small, which meant they would have been overly influenced by Brownian motion. Some drops don’t have complete data sets, indicating they were aborted during the run.

It’s difficult to know today whether Millikan intended to misrepresent his results, though some scientists have examined Millikan’s data and calculated that even if he had included all the drops in his analysis, his measurement for the elementary charge would not have changed much at all.

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Who Did the Oil Drop Experiment?

The Oil Drop Experiment was performed by the American physicist Robert A Millikan in 1909 to measure the electric charge carried by an electron . Their original experiment, or any modifications thereof to reach the same goal, are termed as oil drop experiments, in general.

What is the Oil Drop Experiment?

In the original version, Millikan and one of his graduate students, Harvey Fletcher, took a pair of parallel horizontal metallic plates. A uniform electric field was created in the intermediate space by applying a potential difference between them. The plates were held apart by a ring of insulating material. The ring had four holes, three for allowing light to illuminate the setup, and the fourth one enabled a microscope for viewing. A closed chamber with transparent walls was fitted above the plates.

At the beginning of the experiment, a fine mist of oil droplets was sprayed into the chamber. In modern setups, an atomizer replaces the oil droplets. The oil was so chosen such that it had a low vapor pressure and capable of charging. Some of the oil drops became electrically charged by friction as they forced their way out of the nozzle. Alternatively, charging could also be induced by incorporating a source of ionizing radiation , such as an X-Ray tube, in the apparatus. The droplets entered the space between the plates and raised or fell, according to the requirement, by varying plate voltage.

In terms of the present-day arrangement, when the electric field is turned off, the oil drops fall between the plates under the action of gravity only. The friction with the oil molecules in the chamber makes them reach their terminal velocity fast. The terminal velocity is the constant speed that a freely falling object eventually reaches when the resistance of the medium through which it is falling prevents further acceleration . Once the field is turned on, the charged drops start to rise. This motion happens since the electric force directed upwards is stronger than the gravitational force acting downwards. One charged drop is selected and kept at the center of the field of view of the microscope after allowing all other drops to fall by alternately switching off the voltage source. The experiment is conducted with this drop.

Theory and Calculations

First, the oil drop is allowed to fall in the absence of an electric field, and its terminal velocity, say v 1 , is found out. Using Stokes’ law, the drag force acting on the drop is calculated using the following formula.

Here r is the radius of the drop and ɳ, the viscosity of air.

The weight of the drop, w’, which is the product of its mass and acceleration due to gravity g, is given by the equation,

where ρ is the density of the oil.

However, what we need here is the apparent weight w of the drop in the air given by the difference of the actual weight and the upthrust of the air. We can express w  by the following formula.

Here ρ air denotes the density of air.

When the drop attains terminal velocity, then it has no acceleration. Hence, the total force acting on it must be zero. That means,

The above equation can be used to find out the value of r. Once r is calculated, the value of w can easily be found out from equation (i) marked above.

Now after turning on the electric field between the plates, the electric force F E acting on the drop is,

Where E is the electric field and q the charge on the oil drop. For parallel plates, the formula for E is,

Here V is the potential difference and d the distance between the plates. That implies,

Now if we adjust V to make the oil drop remain steady at a point, then

Thus, the value of q can be calculated.  By repeatedly applying this method to multiple oil droplets, the electric charge values on individual drops were always found to be integer multiples of the smallest value. This lowest charge could be nothing but the charge on the elementary particle, electron. By this method, the electronic charge was calculated to be approximate, 1.5924×10 −19  C, making an error of 1% of the currently accepted value, 1.602176487×10 −19 C. All subsequent research pointed to the same value of charge on the fundamental particle.

Millikan was able to measure both the amount of electric force and magnitude of electric field on the tiny charge of an isolated oil droplet and from the data determine the magnitude of the charge itself. Millikan’s oil drop experiment proved that the electric charge is quantized in nature. The electric charge appears in quanta of magnitude 1.6 X 10 -19 C in oil droplets.

Robert Millikan’s Oil Drop Experiment Animation

Millikan’s oil drop experiment and the atomic theory.

Until the time of the Oil Drop Experiment, the world had little or no knowledge of what is present inside an atom . Earlier experiments by the English Physicist J.J. Thomson had shown that atoms contain some negatively charged particles of masses significantly smaller than that of the hydrogen atom. Nevertheless, the exact value of the charge carried by these subatomic particles remained in the dark. The very existence of these particles was not accepted by many due to a lack of concrete evidence. Thus, the atomic model was shrouded in mystery. In this scenario, with Millikan’s groundbreaking effort to quantify the charge on an electron, the atomic theory came of age in the early years of the twentieth century.

Controversy about the Oil Drop Experiment and Discovery

Robert Millikan was the sole recipient of the Nobel Prize in Physics in 1923 for both his work in this classic experiment and his research in the photoelectric effect . Fletcher’s work on the oil drop project, however, was not recognized. Many years later, the writings of Fletcher revealed that Millikan wished to take the sole credit for the discovery in exchange for granting him a Ph.D. and helping him secure a job after his graduation.

The beauty of the oil drop experiment lies in its simple and elegant demonstration of the quantization of charge along with measuring the elementary charge on an electron that finds widespread applications to this day. With the progress of time, considerable modifications have been made to the original setup resulting in obvious perfection in the results. Still, no substantial deviation from the results of the classical experiment could yet be found.

• Robert Millikan and Harvey Fletcher conducted the oil drop experiment to determine the charge of an electron. The experiment was the first direct and riveting measurement of the electric charge of a single electron.
• They suspended tiny charged droplets of oil between two metal electrodes by balancing downward gravitational force with upward drag and electric forces.
• They later used their findings to determine the mass of the electron.
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Article was last reviewed on Thursday, February 2, 2023

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Millikan's oil drop experiment [duplicate]

In Millikan's oil drop experiment why oil drop only pick negative charge from gas between both charged plates? Why not oil drop pick positive charge?

• experimental-physics

• $\begingroup$ No, both types exist and it will be very easy to see that they are moving in the opposite directions on the microscope. To get a very good data, Millikan's team kept swapping the polarity on the plates so as to keep following the same droplet for hours. That is what led the experiment to give such precision. $\endgroup$ –  naturallyInconsistent Commented Jun 25, 2023 at 9:56
• $\begingroup$ @naturallyInconsistent your statement goes against the wiki article on the first experiment, where the negativeness is stressed en.wikipedia.org/wiki/Oil_drop_experiment .do you have a reference for your statements of both charges being present? $\endgroup$ –  anna v Commented Jun 25, 2023 at 12:00
• 1 $\begingroup$ @annav I don't know. I was referring to the modern replicants thereof, which may have changed the way the charges are induced onto the oil drops. I have done the experiment in an undergrad setting and it was very easy to see that it would always have drops of both charges in my case. $\endgroup$ –  naturallyInconsistent Commented Jun 25, 2023 at 12:45
• $\begingroup$ @naturallyInconsistent if you have X rays in your system then you eventually have lot a electrons and if free electrons are there that means they came from any atom present in atmosphere and if electron detached from atom the atom become positive (cation) ... I'm sorry if I'm wrong I'm new $\endgroup$ –  Abhishek Commented Jun 27, 2023 at 11:46
• $\begingroup$ @Abhishek one would have expected that the X rays kicked an electron from the oil droplet itself and thus made a positive charged droplet rather than negative charged droplet. $\endgroup$ –  naturallyInconsistent Commented Jun 27, 2023 at 11:48

Millikan measured the time for a droplet to fall a particular distance under the influence of gravity and aerodynamic drag, then switched on his electric field and measured its travel time over a similar distance under the influence of gravity, drag, and the field. One source of the precision that allowed him to observe quantization of charge was that he could watch the same droplet for hours, raising and lowering it by turning the electric field on and off, and irradiating the chamber to change the charge on the droplet without changing its size or its drag coefficient. To repeat the experiment with positively charged droplets, he would have needed to reverse the polarity of his battery — a big change to a delicate setup. Otherwise the positive droplets would fall under gravity and fall faster with the field, ending up on the floor in short order.

Nowadays we have inexpensive bipolar DC power supplies, and we can reverse the field on a Millikan-type capacitor pretty easily. But that hadn't been invented when Millikan did his experiment.

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Robert A. Millikan

By Maria Iglesias

• 1 Biography
• 2.1 Oil Drop Experiment
• 2.2 Photoelectric Effect
• 3.1 External links
• 4 References

Robert Andrews Millikan was born March 22, 1868 in Morrison, Illinois. He attended Oberlin College in Oberlin, Ohio, graduating with a degree in classics in 1891. After teaching elementary physics for two years, Millikan went back to school to earn his doctorate in physics from Columbia University. He earned his Ph.D. in 1895, being the first person to do so from that department. Millikan married Greta Ervin Blanchard in 1902. The couple had three children: Clark Blanchard, Glenn Allen, and Max Franklin. In 1908, he became an assistant at the University of Chicago where he later became a professor. Millikan later went on to be the director of the Norman Bridge Laboratory of Physics at the California Institute of Technology in 1921. Robert Millikan died on December 19, 1953 in San Marino, California at the age of 85.

Major Scientific Contributions

Oil drop experiment.

In 1909, Millikan worked with assistant Harvey Fletcher to create the oil drop experiment.The pair were able to find the charge of electron as well as the smallest unit of an electron charged that can be quantized. In this experiment, oil was sprayed onto a plate with a hole. Droplets of oil fell through the whole into a chamber with electrically charged plates. These plates emitted x-rays that caused the negatively charged oil drops to either fall at a slower rate, stop or rise. By comparing the velocity's of the charged and non-charged drops, Millikan and Fletcher found that all the negatively charged particles had a charged that was multiple of 1.6e-19 coloumbs. Millikan won a Nobel Prize in 1923 for his work on this experiment.

Photoelectric Effect

In 1905, Albert Einstein published describing the particle-like qualities of light known as the photoelectric effect. Millikan did not agree with this idea, since previously light had only been described as a wave. To test this Einstein's theory, he created an experiment in light bulb. The machine he designed was a lightbulb with a wheel inside that had 3 cylinders of metals on it: sodium, lithium and potassium. The wheel rotated inside, a metal would be shaved down by a knife and the cylinder was rotated toward a light using an electromagnet. The results concluded that Einstein was correct.

1. Millikan was also known for his talks about the relationship between science and religion.

2. During WWII, he had a large role in designing anti-submarine and meteorological devices.

3. He holds honorary degrees from 25 institutions.

4. Millikan was a tennis enthusiast.

External links

https://www.aip.org/history/gap/Millikan/Millikan.html

http://uudb.org/articles/robertmillikan.html

https://en.wikipedia.org/wiki/Photoelectric_effect

http://www.nobelprize.org/nobel_prizes/physics/laureates/1923/millikan-bio.html

https://en.wikipedia.org/wiki/Oil_drop_experiment

https://en.wikipedia.org/wiki/Robert_Andrews_Millikan

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Science and technologies explained for everyone, physics-millikan’s oil drop experiment.

Robert Andrews Millikan (March 22, 1868 – December 19, 1953) was an American experimental physicist honored with the Nobel Prize for Physics in 1923 for the measurement of the elementary electronic charge and for his work on the photoelectric effect.

In 1897, J. J. Thomson had discovered that the cathode ray are made of sub-atomical, negatively charged particles (which he called “corpuscles”, but was later renamed “electrons”), and measured the charge-to-mass ratio of the electron. However, the actual charge and mass values were unknown. Therefore, if one of these two values were to be measured, the other could easily be calculated. In 1909, Millikan conducted a series of experiments to determine the charge carried by a single electron. He began by measuring the course of charged water droplets in an electric field. The results suggested that the charge on the droplets is a multiple of the elementary electric charge, but the measurement was not accurate enough to be convincing. In 1910, he obtained more precise results with his famous oil-drop experiment in which water (which tends to evaporate too quickly) were replaced with oil.

The Milikan Oil-drop Experiments: This experiment involved observing tiny negatively charged oil droplets between a pair of horizontal metal plates (electrodes). The original apparatus used is shown below.

Figure 1, The original apparatus used by Milikan for the oil-drop experiment.

By Unknown – http://chem.ch.huji.ac.il/~eugeniik /history/millikan.html (taken in 2006, now don’t work),

Public Domain, https://commons.wikimedia.org/w/index.php?curid=685647

First, for a single oil droplet, with zero applied electric field, its terminal velocity was measured. At terminal velocity, the drag force equals the gravitational force, and these depend on the droplet’s radius in different ways, so that the radius of the droplet, and therefore its mass and gravitational force, could be determined with the oil’s known density. Second, an adjustable potential difference (V) was applied between the plates to induce an electric field, and the V was adjusted until the drops were suspended in mechanical equilibrium (electrical force = gravitational force). Now using the known electric field, the oil droplet’s charge could be determined.

Detailed workings:

• The electron’s charge was deduced by Millikan based on his “oil drop” experiment. Figure 2 illustrates the schematic drawing of his experimental set up:

Figure 2,  Millikan’s oil drop experimental set up consists of a parallel-plate capacitor with plates separated by a distance d . The mist of oil droplets sprayed out from the atomizer is electrostatically charged and each oil droplet gains a negative charge of – q .

• In the absence of an electric field, as the oil droplets drift through the hole in the top plate, the force acting on each oil droplet is its own weight and air resistance due to the viscosity of the air. The oil droplets are viewed using a microscope. Millikan measured the mass of an oil droplet by observing its drift velocity as it falls.

At terminal velocity, taking upward direction as positive, the total force F on an oil droplet is: F = 0.

• To suspend a particular oil droplet, the uniform electric field between the plates is varied by varying the potential difference between the plates using the rheostat in the circuit until the downward electric force on the oil droplet is equal to its weight. *

The top plate is kept at a positive potential so that the electric field is acting downwards.

Let the charge of the oil droplet under observation be Q = –q .

Taking upward direction to be positive,

Force on oil droplet due to electric field is F E =-q(-E)=qE

Weight of oil droplet,   W=-mg (weight is acting downwards.)

Resultant force on oil droplet: F=F E +W=qE-mg=0

* Notes:  The oil drop, carrying negative charges, experiences an upward electric force.

Conclusion:

From the above equation, it is possible to calculate the amount of charge on a particular oil droplet. By repeating the experiment for many droplets, Milikan and his pupil Fletcher confirmed that the charges were all small integer multiples of a certain base value, which was found to be 1.592 X 10 −19 C, about 0.6% difference from the currently accepted value of 1.602 X 10 −19 C. They proposed that 1.592 X10 −19 C was the (negative of the) charge of a single electron.

A bit of History:  The oil drop experiment was performed by Robert A. Millikan and his pupil Harvey Fletcher in 1909 to measure the elementary electric charge (an electron’s charge). There are two controversies surrounding this experiment:

• Documents found after Fletcher’s death described how Millikan coerced Fletcher into relinquishing authorship of the discovery as a condition for receiving his PhD.
• Historian Gerald Holton pointed out in 1978 that Millikan recorded more measurements in his journal than he included in his final results, indicating that Milikan selectively omitted a portion of his experimental results in order to present to the scientific community a result with a smaller % error (of 0.6%). If all the results were included, the error would have been 2%. However, according to investigations conducted by David Goodstein, several reasons were provided by Milikan in his more detailed notebooks to account for the failure to generate a complete observation. These include annotations regarding the apparatus setup, oil drop production, and atmospheric effects which invalidated a measurement.

[1] Oil Drop Experiment: https://en.wikipedia.org/wiki/Oil_drop_experiment