Shifting Standards: Experiments in Particle Physics in the Twentieth Century

Chapter 3 “The Isolation of an Ion, a Precision Measurement of Its Charge, and the Correction of Stokes’s Law”

26 • Millikan’s oil Drop Experiments Robert Millikan’s experiments on oil drops (Millikan 1911, 1913) are justly regarded as significant contributions to twentieth-century physics. They established the quantization of electric charge, the existence of a fundamental unit of charge, and they measured that fundamental unit more precisely than had been done previously.1 Although Millikan’s 1913 paper is better known because it provided the most precise and accurate measurement of the charge of the electron, the 1911 paper, which discusses his method in detail, is more suited to our purpose here. Millikan remarked that his method was a considerable improvement on those of previous experimenters: “In a preceding paper (Millikan 1909) a method of measuring the elementary electrical charge was presented that differed essentially from methods used by previous 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 that inhered in preceding determinations made by similar methods such as those 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” (Millikan 1911, 349). Millikan further remarked that the previous methods, each of which used water drops, had several experimental problems. His new method, which used oil drops, had, he claimed, eliminated many of those problems:2 The sources of error which still inhered 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 electric field used; (3) the gradual evaporation of the drops making it impossible to hold a given drop under observation for more than a minute , or to time a drop as it fell under gravity alone through a period of more than CHAPTER 3 “The Isolation of an Ion, a Precision Measurement of Its Charge, and the Correction of Stokes’s Law” Millikan’s Oil Drop Experiments • 27 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 free from all of 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 number of directions . (1911, 350) Millikan began by outlining both the purposes and results of his experiment . These included: 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, the correctness of the view advanced many years ago and supported by the evidence from many sources that all electrical charges, however produced, are exact multiples of one definite, elementary electrical charge. . . . 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 viscosity of air. . . . 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. (Millikan 1911, 350–51)3 Millikan’s apparatus is shown in a photograph in the introduction to this work (see figure I.2) and schematically in figure 3.1. He described it as follows: By means of a commercial “atomizer” a cloud of fine droplets of oil is blown with the aid of dust-free air into the dust free chamber C. One or more of the droplets of this cloud is allowed to fall through a pin-hole p into the space between the plates M, N of a horizontal air condenser and the pin-hole is then closed by means of an electromagnetically operated cover not shown in the diagram. The plates M, N are heavy, circular ribbed brass castings 22 cm. in diameter having surfaces which are ground so nearly to true planes that the error is nowhere more than .02 mm. The planes are held exactly 16 mm. apart by means of three small ebonite posts a held firmly in place by ebonite screws. (351)4 Millikan noted that closing the pinhole eliminated air currents that might cause irregularities and had...