By measuring electrical charges on tiny oil drops, Robert Andrews Millikan determined that the electron is the fundamental unit of electricity.
The first measurement of the electric charge carried by small water droplets was made in 1897 at Cambridge, England. The method timed the rate of fall of an ionized cloud of water vapor inside a closed chamber. The experiment was improved in 1903 through the use of a beam of X rays to produce the cloud between horizontal plates charged by a battery. The rate of descent of the top surface of the cloud between the plates was measured with an electric field switched on and off. The procedure, although an improvement, suffered from instabilities and irregularities on the top of the cloud. The cloud surface was difficult to delineate, with the result being measurements that fluctuated as much as 100 percent.
In 1909, Harvey Fletcher, a young graduate student at the College of Chicago (later the University of Chicago) went to Professor Robert Andrews Millikan to receive suggestions for work on a doctoral thesis in physics. Millikan suggested improving on the measurement of electronic charge previously performed at Cambridge. Millikan’s initial plan was to use an electric field not only strong enough to increase the speed of fall of the upper surface of the ionized cloud but also powerful enough to keep the cloud surface top stationary when the electric field was reversed. This would allow the researcher to observe the rate of evaporation easily and compensate for it in the computations. This technical improvement would permit the researcher, for the first time, to make measurements on isolated droplets and eliminate the experimental uncertainties and assumptions involved in using the cloud method.
Millikan’s improvement included the construction of a 10,000-volt small cell storage battery with enough strength to hold the top surface of the cloud suspended long enough to allow the measurement of the rate of evaporation of the droplets. When the electric field was turned on, however, the result was a complete surprise to Millikan. The top of the cloud surface instantaneously dissipated, and, because the experimental result assumed a rate of fall for the ionized cloud, Millikan saw this result as a complete failure.
Repeated tests showed that whenever the cloud was dispersed, a few droplets remained. By nature, however, these droplets had the proper charge-to-mass ratio to allow the downward force of gravity or weight of the droplet to be balanced by the upward pull of the electric field on the droplet’s charge. This procedure became known as the “balanced drop method.”
The actual experimental arrangement that Millikan and Fletcher used consisted of a small box with a volume of 2-3 cubic centimeters (0.12-0.18 cubic inches) fastened to the end of a microscope. A tube extended from the box to an expansion chamber secured by an adjustable petcock valve that allowed a rapid expansion of air to form a water-vapor cloud in the box. On the ends of the box were two brass conducting plates about 20 centimeters (7.87 inches) in diameter and 4 millimeters (0.16 inch) thick. A small hole was bored into the top plate to allow the oil mist from an atomizer to enter the region between the two plates, which were separated by approximately 2 centimeters (0.79 inch). A small arc light with two condensing lenses created a bright narrow beam that was in turn permitted to pass between the plates.
An instrument called a cathetometer was placed on the microscope so that the microscope could be raised or lowered to the proper angle with the light beam for best illumination (which from practice turned out to be about 120 degrees). The plate separation allowed the researchers to apply a potential difference and produce an electric field. They operated the apparatus by turning on the light, focusing the microscope (which was placed about 1 meter, or 3.28 feet, from the plates), and then spraying oil over the top plate while switching on the battery. When viewed through the microscope, the oil droplets appeared like “little starlets” that had the colors of the rainbow.
Millikan and Fletcher noticed that when the electric field was first switched on, the droplets would move at different speeds; some moved slowly upward whereas others moved downward more quickly. Superimposed on the droplets’ downward fall was a small random back-and-forth motion (known as Brownian movement) caused by the collisions of the tiny droplets with thermally agitated air molecules within the chamber. When the researchers reversed the electric field by changing the polarity of the battery, the same droplets that had been moving downward moved upward, and vice versa. They deduced that the nature of this motion indicated that some of the droplets were negatively charged, whereas the others carried a positive charge.
Through the timely application of a polarity to the electric field, they were able to keep selected droplets in the field of view for longer periods of time to obtain values for the calculation of electronic charges. For this condition, the electric field interacting with the charge on the droplet created an upward force that compensated only for the weight of the droplet or the downward force. The electronic charge calculation depended on a suitable balance between the intensity of the electric field and the amount of electrical charge on the droplet that overcame its weight.
One major experimental problem remained, however; the water composing the droplet evaporated so quickly that visibility was initially limited to only about two seconds. After some discussion about this problem, Millikan and Fletcher substituted several other substances, including mercury and oil. Oil had an advantage, as it was easy to obtain and to handle, and its rate of evaporation was much slower than that of water. In time, the researchers refined the experiment to obtain greater precision. The metal plates were machined more accurately, and the air between the plates was enclosed to prevent air drafts. Also, X-ray and radium sources were aimed into the chamber, producing greater ionization and more charged droplets than an atomizer could produce.
From their examination of the smallest experimental values obtained, it became apparent to Millikan and Fletcher that the charges on tiny oil droplets occur only in multiples of the smallest possible charge; no fractional amount of this basic charge was ever observed—only whole-number increments. This implied that the unit charge obtained could not be subdivided into smaller charges and was independent of the droplet size. These exact values showed that the electronic charge was not merely a statistical mean, as previous experimenters believed. The experiment, in fact, provided direct evidence for the existence of the electron as a finite-sized particle carrying a fundamental charge. It also enabled researchers to examine the attractive or repulsive properties of isolated electrons and to determine that electrical phenomena in solutions and gases are caused by electrical units that have fundamentally the same charge.
The oil-drop experiment was an improvement over previous measurements in that Millikan was able to control the strength of the electric field with accuracy while varying the droplet size. He also demonstrated that the oil droplet when completely discharged fell at the same rate as an uncharged droplet with the electric field on. This indicated that something fundamental, which Millikan chose to call electricity, could be placed on or removed from the droplet only in exact amounts. Reversing the electric field to allow it to pull the droplets upward rather than downward permitted the researcher to freeze the motion of single droplets, giving more precise charge calculations than the method of trying to follow whole cloud motion, which was based only on statistical methods and could not give exact numbers.
As a result of Millikan’s determination of the absolute charge on the electron and the previously known ratio of charge to mass, combined with the knowledge of the exact charges on ionized atoms from previous positive-ray analysis or electrolysis, the absolute masses of both the electron and the atom could be determined with great precision. With knowledge of the charge on the electron, a new unit of energy—the electronvolt—could be defined. The kinetic energy of particles of unit charge that had moved through a potential difference now could be computed with the known mass of the particle entering the equation.
Another outcome of the oil-drop experiment was the calibration of a correction factor used for Stokes’s law.
-
Fletcher, Harvey. “My Work with Millikan on the Oil-Drop Experiment.” Physics Today 44 (June, 1982): 43-47. In this reminiscence, Fletcher relates his experiences while working with Millikan. Photographs, a diagram, and a detailed description of how the experiment was performed will interest the lay reader. -
Fraser, Charles G. Half-Hours with Great Scientists: The Story of Physics. Toronto: University of Toronto Press, 1948. Informative descriptions for a general audience of the great discoveries in physics. The chapter titled “Story of Electricity and Magnetism” provides not only a short summary of experimental results but also discussion of how Millikan’s work related to previous efforts. -
Heathcote, Niels Hugh de Vaudrey. “Robert Andrews Millikan.” In Nobel Prize Winners in Physics, 1901-1950. New York: Henry Schuman, 1953. This chapter on Millikan is organized into three sections: a biographical sketch, a description of the prizewinning work, and a summary of Millikan’s contribution to science. Provides an informative description of the experiment along with diagrams of the oil-drop apparatus. -
Millikan, Robert A. Autobiography. 1950. Reprint. New York: Arno Press, 1980. Millikan describes his life from early childhood and education through his later work on projects at the California Institute of Technology. An entire chapter is devoted to the oil-drop experiment, with tables, procedures used, and a detailed illustration. This book is a must for the lay reader who desires more than a cursory summary of Millikan’s work. -
Niaz, Mansoor, and María A. Rodríguez. “The Oil Drop Experiment: Do Physical Chemistry Textbooks Refer to Its Controversial Nature?” Science & Education 14 (January, 2005): 43-57. The authors of this article argue that Millikan’s oil-drop experiment was (and still is) difficult to perform and generated considerable controversy in the early twentieth century, yet this information is rarely mentioned in modern-day physical chemistry textbooks. The article reports on their findings from an evaluation of the discussions of the experiment in twenty-eight such texts. -
Oldenberg, Otto. Introduction to Atomic and Nuclear Physics. 3d ed. New York: McGraw-Hill, 1961. Many physics texts provide brief descriptions of Millikan’s experiment, but few explain the technique and results as well as this one does. Readers with a basic background in mathematics should be able to understand this book. -
Romer, Alfred. “Robert A. Millikan, Physics Teacher.” Physics Teacher 78 (February, 1978): 78-85. A unique view into the character of Millikan from the perspective of a graduate student who knew him. Provides a brief description of the oil-drop experiment and an example of Millikan’s insight and originality.
Geiger and Rutherford Develop a Radiation Counter
Hess Discovers Cosmic Rays
Millikan Investigates Cosmic Rays
Anderson Discovers the Positron