Chronology of Milestone Events in Particle Physics - MILLIKAN 1911
Chronology of Milestone Events in Particle Physics

  Nobel prize to R. A. Millikan awarded in 1923 "for his work on the elementary charge of electricity and on the photo-electric effect''  

MILLIKAN 1911

Millikan, R.A.;
The Isolation of an Ion, a Precision Measurement of its Charge, and the Correction of Stokes's Law
Phys. Rev. XXXII (1911) 349;

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Introduction
In a preceding paper a method of measuring the elementary electrical charge was presented which differed essentially from methods which had been used by earlier observers only in that all of the measurements from which the charge was deduced were made upon one individual charged carrier. This modification eliminated the chief sources of uncertainty which inhered in preceding determinations by similar methods such as those made by Sir Joseph Thomson, H. A. Wilson, Ehrenhaft, and Broglie, all of whom had deduced the elementary charge from the average behavior in electrical and gravitational fields of swarms of charged particles.
The method used in the former work consisted essentially in catching ions by C. T. R. Wilson's method on droplets of water or alcohol, then isolating by a suitable arrangement a single one of these droplets, and measuring its speed first in a vertical electrical and gravitational field combined, then in a gravitational field alone.
The sources of error or uncertainty which are still inherent in the method arose from:
(1) the lack of complete stagnancy in the air through which the drop moved;
(2) the lack of perfect uniformity in the electrical field used;
(3) the gradual evaporation of the drops, rendering it impossible to hold a given drop under observation for more than a minute, or to time the drop as it fell under gravity alone through a period of more than five or six seconds;
(4) the assumption of the exact validity of Stokes's law for the drops used.
The present modification of the method is not only entirely free from all these limitations, but it constitutes an entirely new way of studying ionization and one which seems to be capable of yielding important results in a considerable number of directions.
With its aid it has already been found possible:
1. To catch upon a minute droplet of oil and to hold under observation for an indefinite length of time one single atmospheric ion or any desired number of such ions between 1 and 150.
2. To present direct and tangible demonstration, through the study of the behavior in electrical and gravitational fields of this oil drop, carrying its captured ions, of the correctness of the view advanced many years ago and supported by evidence from many sources that all electrical charges, however produced, are exact multiples of one definite, elementary, electrical charge, or in other words, that an electrical charge instead of being spread uniformly over the charged surface has a definite granular structure, consisting, in fact, of an exact number of specks, or atoms of electricity, all precise alike, peppered over the surface of the charged body.
3. To make an exact determination of the value of the elementary electrical charge which is free from all questionable theoretical assumptions and is limited in accuracy only by that attainable in the measurement of the coefficient of viscosity of air.
4. To observe directly the order of magnitude of the kinetic energy of agitation of a molecule, and thus to bring forward new direct and most convincing evidence of the correctness of the kinetic theory of matter.
5. To demonstrate that the great majority, if not all, of the ions of ionized air, of both positive and negative sign, carry the elementary electrical charge.
6. To show that Stokes's law for the motion of a small sphere through a resisting medium, breaks down as the diameter of the sphere becomes comparable with the mean free path of the molecules of the medium, and to determine the exact way in which it breaks down.
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Accelerator NONE Detectors OTHER

Related references
More (earlier) information appears in
R. A. Millikan, Science 32 (1910) 436;
R. A. Millikan, Phil. Mag. 19 (1910) 209;
See also
J. J. Thomson, Phil. Mag. 5 (1903) 346;
H. A. Wilson, Phil. Mag. 5 (1903) 429;
F. Ehrenhaft, Phys. Zeitshr., (Mai 1909), (July 1910);
L. de Broglie, Le Radium, (July 1909);
J. J. Thomson, Phil. Mag. 46 (1898) 528;
J. J. Thomson, Phil. Mag. 48 (1899) 547;

Particles studied
  e qn

Record comments
First conclusive measurement of the charge of the electron.
    
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