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Milliken's Oil Drop Experiment

The Millikens Oil Drop Experiment was an experiment performed by Robert A. Millikan and Harvey Fletcher  in 1909 to measure the charge of an electron. This experiment proved to be very crucial in the physics community.

Millikens Oil Drop Experiment Definition

In the experiment, Milliken allowed charged tiny oil droplets to pass through a hole into an electric field. By varying the strength of the electric field the charge over an oil droplet was calculated, which always came as an integral value of ‘e.’

Apparatus of the Milliken’s Oil Drop Experiment

The apparatus for the experiment was constructed by Milliken and Fletcher. It incorporated two metal plates held at a distance by an insulated rod. There were four holes in the plate, out of which three were there to allow light to pass through them and one was there to allow viewing through the microscope.

Ordinary oil wasn’t used for the experiment as it would evaporate by the heat of the light and so could cause an error in the Millikens Oil Drop Experiment. So, the oil that is generally used in a vacuum apparatus which is of low vapour pressure was used.

Milliken’s Oil Drop Experiment Procedure

• Oil is passed through the atomizer from where it came in the form of tiny droplets. They pass the droplets through the holes present in the upper plate of the apparatus.
• The downward motions of droplets are observed through a microscope and the mass of oil droplets, then measure their terminal velocity.
• The air inside the chamber is ionized by passing a beam of X-rays through it. The electrical charge on these oil droplets is acquired by collisions with gaseous ions produced by ionization of air.
• The electric field is set up between the two plates and so the motion of charged oil droplets can be affected by the electric field.
• Gravity attracts the oil in a downward direction and the electric field pushes the charge upward. The strength of the electric field is regulated so that the oil droplet reaches an equilibrium position with gravity.
• The charge over the droplet is calculated at equilibrium, which is dependent on the strength of the electric field and mass of droplet.

Milliken’s Oil Drop Experiment Calculation

F up = F down

F up = Q . E

F down = m.g

Q  is  an  electron’s  charge,  E  is  the  electric  field,  m  is  the  droplet’s  mass,  and  g  is  gravity.

One can see how an electron charge is measured by Millikan. Millikan found that all drops had charges that were 1.6x 10 -19 C multiples.

Milliken’s Oil Drop Experiment Conclusion

The charge over any oil droplet is always an integral value of e (1.6 x 10 -19 ). Hence, the conclusion of  Millikens Oil Drop Experiment is that the charge is said to be quantized, i.e. the charge on any particle will always be an integral multiple of e.

Frequently Asked Questions – FAQs

What did millikan’s oil drop experiment measure.

Millikan oil-drop test, the first simple and persuasive electrical charge calculation of a single electron. It was first conducted by the American physicist Robert A. in 1909. He discovered that all the drops had charges that were simple multiples of a single integer, the electron’s fundamental charge.

What is the importance of Millikan’s oil drop experiment?

The experiment with Millikan is important since it defined the charge on an electron. Millikan used a very basic, very simple system in which the behaviour of gravitational, electrical, and (air) drag forces were controlled.

What did Millikan conclude after performing his oil drop experiment?

An integral multiple of the charge on an electron is the charge on every oil decrease. About an electric force. In a relatively small amount, the charge and mass of the atom must be condensed.

Why charges are quantized?

Charges are quantized since every object’s charge (ion, atom, etc.) Charge quantization, therefore, implies that no random values can be taken from the charge, but only values that are integral multiples of the fundamental charge (proton / electron charge).

Can charge be created or destroyed?

The Charge Conservation Law does not suggest that it is difficult to generate or remove electrical charges. It also means that any time a negative electrical charge is produced, it is important to produce an equal amount of positive electrical charge at the same time so that a system’s overall charge does not shift.

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The Millikan Oil Drop Experiment

Introduction To The Millikan Oil Drop Experiment

In this article, you will learn all you need to know (and more) about the Millikan Oil Drop Experiment. If you like this article, check out our other articles!

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Who was Robert A. Millikan?

Robert A. Millikan was born on the 22nd of March, 1868 in Illinois, (U.S.A.). Growing up, Millikan spent most of his childhood living in a rural town called Morrison. Then, in 1875, his family relocated to Maquoketa, Iowa where Millikan started attending Maquoketa high school. Millikan excelled in his learning and decided to further his studies by attending Oberlin College in Ohio. During this time, Millikan started teaching a physics class and decided to pursue the subject as a career. He later obtained his Ph.D. in physics from Columbia University.

After graduating from Columbia, Millikan traveled to the universities of Berlin and Göttingen. There, he furthered his knowledge within his field before returning to the United States to be an assistant at Chicago University’s Ryerson Laboratory. During his time there, Millikan authored (and co-authored) several physics textbooks. Eventually, in 1907, a research project of Millikan’s led to the development of the Oil Drop Experiment .

The Experiment

Devised by Robert A. Millikan and Harvey Fletcher, the Millikan Oil Drop Experiment is conducted in a chamber and is a method of measuring the electric charge of a single electron .

To elaborate, this chamber contains an atomizer, a microscope, a light source, and two parallel metal plates. These metal plates obtain a negative and a positive charge when an electric current would pass through them.

The Procedure

First, the atomizer was to release a fine mist of oil that would drift within the chamber. While drifting, the droplets of oil would make their way into the bottom half of the chamber (between the metal plates) due to a gravitational pull. Here, the oil droplets would be ionized into being negatively charged. Thereafter, while these negatively charged droplets are being pulled down by gravity, the external power-dial would be used to add a charge to the two metal plates (above and below the droplets). Specifically speaking, the  top  plate would cultivate a  positive  charge, and a  negative  charge would be cultivated on the  bottom  plate.

This creates a situation in which the oppositely charged (positive) metal plate is pulling the negatively charged droplet upwards , while gravity is pulling the droplet downwards . Or in other words, the electrostatic and gravitational forces are now controlling the direction in which the droplet is flowing. Now, if the electrostatic force is greater, then the droplet would rise towards the positively charged plate. Likewise, if the gravitational force is greater than the electrostatic force, then the droplet would be pulled down.

Observations and Conclusion

The purpose of this experiment was to balance these two electrostatic and gravitational forces – which would cause the droplets to halt midair. By doing this, the droplet’s mass, gravitational force, and electrostatic force could be measured, revealing the charge of the electron. Furthermore, by doing these final calculations, Millikan was able to reveal that the charge of an electron would be multiples of  1.602×10−19 Coulombs .

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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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The Millikan Oil Drop Experiment

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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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Landmarks —Millikan Measures the Electron’s Charge

Landmarks articles feature important papers from the archives of the Physical Review journals.

Researchers now routinely isolate single electrons in quantum dots, but a century ago the state-of-the-art charge-trapping device was a droplet of clock oil. Robert Millikan’s oil drop experiment provided the first clear measurement of the fundamental electric charge and thus helped cement the notion that nature is “grainy” at the smallest level. The first results came out in 1910, but the seminal work was a 1913 paper in the Physical Review . Millikan reported a value for the fundamental electric charge that was within half a percent of today’s accepted value. The experiment helped earn Millikan a Nobel prize in 1923 but has been a source of some controversy over the years.

J. J. Thomson discovered the electron in 1897 when he measured the charge-to-mass ratio for electrons in a beam. But the value of the charge and whether it was fundamental remained open questions. Thomson and others tried to measure an irreducible electric charge by looking at clouds of water droplets. Using various techniques, they estimated the smallest charge that a droplet could hold, but the results were not entirely convincing because they relied on averages over many particles of various sizes. “The evidence for a unitary charge was at the time very ambiguous,” says science historian Gerald Holton of Harvard University.

At the University of Chicago in the 1900s, Millikan and his graduate students realized that ramping up the electric field would disperse a water cloud, so that only a few droplets remained. He decided to try isolating single droplets, but it soon became clear that single water droplets evaporated too quickly to make reliable measurements. One of his students, Harvey Fletcher, found that long-lasting droplets could be made with a light oil that was used for lubricating clocks.

The oil drop experiment that Millikan and Fletcher designed had two chambers. In the upper chamber, an atomizer (like that used in perfume bottles) dispersed a fine mist of micron-sized oil droplets. Individual droplets would fall through a pinhole into the lower chamber, which consisted of two horizontal plates, with one held 16 millimeters above the other. The air in this chamber was ionized with x rays , so that ions or free electrons could be captured on the falling droplets. A small window on the side allowed the scientists to observe the droplets through a telescope. The droplets fell slowly enough—due to atmospheric drag—that the researchers could measure their downward speed by eye, using horizontal lines in the telescope. From this speed, they could estimate the size and mass of each droplet.

They then applied a high voltage across the plates and measured the upward speed of the droplet, to determine the electric force and ultimately the charge. Multiple measurements on a single droplet could be performed by repeatedly turning the electric field on and off. The droplets had various amounts of charge on them (and they would often gain or lose charge during an observation), but the data showed that the charge was indeed quantized into integer multiples of a unit charge.

In 1910 Millikan published the first results of these experiments [1] (Fletcher was not included as an author, based on a deal the two struck [2] ). Millikan then made several improvements, including an empirical estimate of the drag forces. The culmination of this effort, reported in 1913, was a value of the fundamental charge with an error bar of just 0.2 percent. The precision acquired was so great that “other experiments did not improve on his result until a decade later,” Holton says.

But Felix Ehrenhaft of the University of Vienna repeatedly challenged Millikan’s results, based on his own measurements of “sub-electron” charges on small metal particles. The dispute lasted for many years—known as the “Battle over the Electron”—but eventually most physicists sided with Millikan.

In more recent years, historians who have examined Millikan’s lab notes have said that he discarded some of the measurements to boost the evidence of a fundamental charge. But David Goodstein of the California Institute of Technology in Pasadena believes these accusations of fraud are unwarranted. He has analyzed the notes and says that Millikan excluded droplets because their observations were incomplete, not because their implied charge didn’t match his expectations [3] . “Millikan’s oil drop experiment is a classic example of outstanding physics done by one of the giants of his era,” Goodstein says.

–Michael Schirber

Michael Schirber is a Corresponding Editor for  Physics Magazine based in Lyon, France.

• R. A. Millikan, “The Isolation of an Ion, a Precision Measurement of its Charge, and the Correction of Stokes’s Law,” Science 32 , 436 (1910) ; first reported at the American Physical Society meeting, 23 April 1910, Phys. Rev. (Series I) 30 , 656 (1910)
• H. Fletcher, “My work with Millikan on the oil‐drop experiment,” Phys. Today 35 , 43 (1982)
• D. Goodstein, “In Defense of Robert Andrews Millikan,” Am. Sci. 89 , 54 (2001)

More Information

Focus story on Millikan’s measurement of Planck’s constant

article by Gerald Holton on the Millikan-Ehrenhaft Dispute

article about the ethics of Millikan’s handling of data

Millikan Nobel Prize: Nobel lecture, biography, and other information

On the Elementary Electrical Charge and the Avogadro Constant

R. A. Millikan.

Phys. Rev. 2 , 109 (1913)

Published August 1, 1913

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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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When the real penguins went extinct, openmind books, scientific anniversaries, cooling paints to get more sustainable buildings, featured author, latest book, millikan, the first physicist to see the electron.

In 1923, the American physicist  Robert Andrews Millikan  (1868-1953) was awarded the  Nobel Prize for Physics ,

“for his work on the elementary charge of electricity and on the photoelectric effect”.

In his Nobel Lecture  “The Electron and the Light-Quant from the Experimental Point of View” he referred to the  experiment  that had enabled him determine the charge of the electron, leaving his audience convinced that he had seen electrons:

“He who has seen that experiment, and hundreds of investigators have observed it, has literally seen the electron”.

Quite often we physicists say that we  see  those things on which we are working, no matter how small, or even abstract they may be. There is little doubt, however, that inside the apparatus used by Millikan there was a world of particles  with which he became so familiar that he unblushingly claimed to  see  things in that  world .

Physics requires experiments, accurate measurement and, of course, conclusions to be drawn. There are numerous examples of decisive experiments in the history of physics. One of the most famous and important of these was that which enabled  the determination of the charge of the electron, conducted by Millikan in 1909, which has  become known as the  oil-drop experiment  or simply  Millikan’s experiment. Indeed, it is considered to be one of “most beautiful experiments in physics” and was pivotal in enabling the measurement of the  charge of the electron

Robert Andrews Millikan  was born in Morrison, Illinois (USA) on March 22, 1868 . After graduating from  Oberlin College in Ohio (1891) -where he particularly enjoyed studying Greek and mathematics- he did two courses in elementary physics, which awakened his interest in this discipline. In 1893 he was awarded a  fellowship  at Columbia University, from which he received his PhD in 1895 for  a thesis on the polarization of light emitted by incandescent surfaces.  A   phenomenon that had originally been observed (1824) by  François Aragó , Millikan used molten gold and silver from the US Department of Treasury to prove his thesis. After spending a year (1896) in Germany at the Universities of Berlin and Götingen, he return to the United States to take up an invitation from the physicist and fellow Nobel Laureate Albert A. Michelson  to become his assistant in the recently founded  Ryerson Laboratory  at the University of Chicago. He was eventually to become a lecturer there (1910), a post he held until 1921. In the course of his life (he died in 1953) Millikan was a Professor of Physics, Director of the Norman Bridge  Physics Laboratory and President of the  California Institute of Technology   (CALTECH).

A drop of oil to “unmask” the electron

Millikan was a key figure in the development of physics in the United States in the first half of the 20th century. If required to classify him as a physicist, his facet as an experimental physicist  would undoubtedly have to be highlighted, as would the numerous important discoveries he made, predominantly in the fields of optics and molecular physics.  Millikan’s first great achievement was to determine the charge of the electron, to which end he used the “oil-drop method”.   J. J. Thomson , the British physicist, had already established the charge-to-mass ratio of the electron back in 1897, but neither of them separately. Accordingly, if it was possible to determine one of these values separately (charge or mass), the other could be easily calculated. Millikan, with the aid of  Harvey Fletcher , one of his doctoral students, used the oil-drop experiment to measure the charge of the electron (and with this, its mass). When Millikan began a long series of experiments in 1907, he had already been working at the University of Chicago for ten years, had got married, was the father of three children and about to celebrate his fortieth birthday. He had earned great renown as a physics lecturer, but had still not achieved anything noteworthy as a scientific investigator.

The  fundamental electric charge  is one of the basic constants in physics, consequently, its accurate determination is essential for this discipline. In his experiment, Millikan measures the electrical force on a small oil-drop that has been charged by an electrical field created between two electrodes when the drop was in the gravitational field. As the electrical field was known, it was possible to determine the accumulated charge on the oil-drop.

A  spray  formed oil-drops, some of which fell through a small gap into a uniform, electrical field area space by two parallel, charged plates. A microscope made it possible to observe a particular oil-drop and learn its mass by measuring the terminal speed of its fall. The oil-drop was charged with x-rays, and by adjusting the electrical field, it was possible to get it to remain in repose, in static equilibrium, when the electrical force was equal to the opposing gravitational force. Millikan, carrying out a long and tedious task that involved a set of collateral experiments, repeated the experiment numerous times, eventually concluding that the results obtained could be explained if there was a single, elementary charge (the value of which he determined) and the charges identified were integer multiples of this number.

In 1909 he sent his first article for publication, in which he explained a technique he called  “a drop equilibrium method to determine the charge of the electron, e”, entitled   “A new modification of the cloud method of determining the elementary electrical charge and the most probable value of that charge”  .  Millikan included his personal opinions on the reliability and validity of each one of his 38 observations. He marked seven “very good” observations with two asterisks, ten “good” ones with a single asterisk and left the remaining thirteen “satisfactory” ones unmarked. A genuine act of honesty, with respect to which the science historian  Gerald Holton would later refer to as  “a rather uncommon gesture in scientific publications”.  In September 1910, Millikan published a second article in the journal  Science  about the charge of electrons entitled,   “The isolation of an ion, a precision measurement of its charge, and the correction of Stokes’s law”   , the first ever to fully explain his “drop equilibrium” method. Three years later, in 1913 Millikan improved  the results obtained to determine the charge of the electron  e  = 4.774 ± 0.009 x 10 -10  electrostatic units of charge (esu), i.e. 1.592 x 10 -19  C (1 esu = 3.33564×10 -10  C) which is slightly below the currently accepted value of 1.602 x 10 -19  C, most likely because Millikan used an inaccurate value for the viscosity of the air.

An unexpected ally for Einstein

But this was not the only “crucial” experiment conducted by the meticulous Millikan. In 1905, in the course of his  Annus Mirabilis ,  Albert Einstein   published the article entitled,  “On a Heuristic Point of View about the Creation and Conversion of Light” . In this article Einstein theoretically analyzes the photoelectric effect, convincingly introducing the concept of the  “quantum of light”  (later re-named the  photon ) and applying the ideas  Max Planck , before any of his colleagues had done so, to theoretically explain the photoelectric effect. Planck himself turned out to be one of the staunchest critics of this idea of  light quanta , while  Millikan dismissed Einstein’s idea as a “rash, not to say foolish hypothesis”, quickly getting down to work on experimentally showing Einstein the error of his ways. After ten years of experiments (1916), Millikan published his results in the journal  Physical Review  in an article entitled,  “A Direct Photoelectric Determination of Planck’s h”  .  Nevertheless, and contrary to what he had originally intended, Millikan not only experimentally validated Einstein’s equation for the photoelectric effect, but also determined  Planck’s constant , h. The conclusion of Millikan’s 1916 article leaves no room for doubt:

“Einstein’s photoelectric equation has been subject to very searching tests and it appears in every case to predict exactly the observed results”.

Nonetheless, Millikan intended to  demonstrate with his experiments that Einstein’s idea of “light quanta” was wrong.  In an article entitled,  “Albert Einstein on His Seventieth Birthday” (1949), in the journal  Reviews of Modern Physics , Millikan wrote:

“I spent ten years of my life testing that 1905 equation of Einstein’s [the photoelectric effect], and contrary to all my expectations I was compelled in 1915 to assert its unambiguous experimental verification in spite of its unreasonableness, since it seemed to violate everything that we knew about the interference of light”.

On November 14, 1923 Millikan received a  telegram  from the Royal Swedish Academy of Sciences informing him that he had been awarded the  Nobel Prize for Physics   for his work on the elementary charge of electricity and on the photoelectric effect , thus making him the second American to receive it. The text of the telegram reads as follows:

5 gs d 2625 803R

STOCKHOLM 1030 AM NOV 14 1923

DOCTOR MILLIKAN

PASADENA CALIF

NOBEL PRIZE FOR PHYSICS AWARDED TO YOU PLEASE WIRE WHETHER YOU CAN BE PRESENT AT STOCKHOLM DEC 10 TH

HOEDERBAUM, SECY ACADEMY OF SCIENCE

Millikan took part in the Third Solvay Conference  held in Brussels (1921) and was  awarded   Honorary Doctorates by twenty-five universities around the world . In addition to the Nobel Prize for Physics, he was also awarded the Comstock Prize in Physics by the National Academy of Sciences, the Edison Medal and the Hughes Medal by the  Royal Society , not to mention a host of other honors. Millikan died on  December 19, 1953  in San Marino, California, at the age of 85.

Augusto Beléndez

Professor of Applied Physics at the University of Alicante and member of the Spanish Royal Physics Society

Bibliography

Azcárraga, J. A., “The Legacy of Albert Einstein (1879-1955)”, OpenMind, November 20, 2015.

Beléndez, A., “Einstein 1905: From ‘Energy quanta’ to ‘Light quanta’”, IYL2015-Blog, November 23, 2015.

Crease, R. P.,  The Prism and the Pendulum. The ten most beautiful experiments in Science  (Penguin, 2004).

Doménech, F., “Max Planck, the Messiah of quantum physics”, OpenMind, April 23, 2016.

Sánchez Ron, J. M.;  Historia de la Física Cuántica I. El período fundacional  (1860–1926) (Crítica, Barcelona, 2001).

“August, 1913: Robert Millikan Reports His Oil Drop Results”. This Month in Physics History. APS News, August/September 2006 (Volume 15, Number 8).

“The making of Caltech’s first Nobel: Robert Millikan’s road to Stockholm” (The Caltech Archives).

“Robert A. Millikan – Biographical”.  Nobelprize.org.  Nobel Media AB 2014. Web. April 12, 2017.

Robert Andrews Millikan, Wikipedia (Consulted April 16, 2017).

Related publications

• Einstein’s Miracle Year
• When Einstein “Saw the Light”
• Max Planck, the Messiah of quantum physics

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Robert Millikan

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• The Nobel Prize - Biography of Robert A. Millikan
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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.

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• Published: 26 September 2023

In retrospect

Nobel 1923: determining the charge of the electron

• Ankita Anirban 1  

Nature Reviews Physics volume  5 ,  page 553 ( 2023 ) Cite this article

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One hundred years ago, the Nobel Prize in Physics was awarded to Robert Millikan for his work on the elementary charge of electricity and on the photoelectric effect . Although he also verified Einstein’s work on the photoelectric effect, it is for determining the charge of the electron that Millikan is best known for today. He placed the value of e at 1.5924 × 10 –19 C. Millikan’s value was an underestimate of 0.6%, but such was the reverence for his work that it took years of experiments to settle on the currently accepted value of 1.6021 × 10 –19 C. As Richard Feynman quipped in the 1970s, “when they got a number that was too high above Millikan’s, they thought something must be wrong — and they would look for and find a reason why something might be wrong. When they got a number close to Millikan’s value, they didn’t look so hard.”

The turning point came in 1908 with the idea of replacing water with oil. The historical record is unclear on exactly whose idea this was. But very quickly, Millikan’s graduate student, Harvey Fletcher, set up a new apparatus (pictured), with a fine mist of oil suspended between the two electric plates. In a Physics Today article published in 1982, Fletcher vividly describes looking through his telescope at the oil drops between the plates and seeing “little starlets, having all colours of the rainbow”: his first observation of Brownian motion. By the end of the day, Fletcher had an estimate for the value of e . Over the next two years, Millikan and Fletcher painstakingly measured the rise and fall of different sized oil droplets, and how they were affected by the electric field. By 1910, two big results had come out from this experiment: the value of e , and the value of Ne , where N is Avogadro’s number, which was measured based on the Brownian motion observations.

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Original article

Millikan, R. A. XXII. A new modification of the cloud method of determining the elementary electrical charge and the most probable value of that charge. Lond. Edinb. Dublin Philos. Mag. J. Sci. 19 , 209–228 (1910)

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Millikan, R. A. On the elementary electrical charge and the Avogadro constant. Phys. Rev. 2 , 109–143 (1913)

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Holton, G. in Historical Studies in the Physical Sciences (eds McCormmach, R. et al.) 161–224 (Johns Hopkins Univ. Press, 1978)

Fletcher, H. My work with Millikan on the oil-drop experiment. Phys. Today 35 , 43–47 (1982)

Goodstein, D. In defense of Robert Andrews Millikan. Am. Sci. 89 , 54–60 (2001)

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