The Nobel Prize of 1923 Morris Bader Morauian College Bethlehem, Pennsyluania 18018 By the turn of the twentieth century all of science was committed to the atomic and corpuscular view of nature. The discoverv of radioactivitv then led to the idea of sub-atomic particles" In 1881 Stone; had coined the word "electron" as a particle of negative charge (1). This was sheer speculation since no one had seen evidence of an isolated electron. As the mass of the electron was so verv small, the charge provided the more useful property to determine whether itwas a discrete "atomic" particle or not. The only reasonable prior value came fro'm dividing Faraday's constant by Avogadro's number from which the average charge e came to be 4.8 X 10-lo esu. As Avogadro's number was known to be in error by at least lo%, it still left much doubt as to the atomicity of the electron. A maior attack on the nroblem heean in 1897 in the Cavendish laboratories. In his famous cloud chamber experiment C. T. R. Wilson had discovered how to make the tracks of radioactive particles visible as they tore through the air by causing a supersaturated vapor to condense on the ions left behind. Using the cloud chamber J. J. Thomson attempted to measure the total charge on a small cloud, but had little success (2). H. A. Wilson had the idea of creating a vertical electrical field by placing a pair of horizontal metal plates in the chamber (3). First, a charged cloud was allowed to fall by gravity alone, and the velocity was measured. Then an identical ehareed cloud was formed. but now the metal olates were ~~~-~~ ~~~~~w~ electrified with high voltage tdretard the fall. In this way the charee of the cloud could he determined. But clouds are tenuousthiugs to work with, and no two are really alike. Wilson recognized very early that his results were quite inaccurate. In 1907 K. Przibram of the Univelsity of Vienna duplicated the Wilson apparatus but thought that the difficulty lay with the alcohol vapor used to form the cloud ( 4 , 5 ) .He tried everything from organics to phosphoric acid mist. Reporting hundreds of determinations his values of e ranged from 2 to 25 X 10-'0 esu. Przibram's work only seemed to support previous results that electrons varied in charge, a view which contradicted most of scientific theory and intuition. In spite of this terrible record of cloud work Robert A. Millikan of the University of Chicago also hegan to repeat the work of H. A. Wilson. Although this was his first experimental pursuit after fifteen years of teaching, his initial reported results in 1909 were more consistent than anv obtained before (6, 7). At this same time, Felix Ehrenhaft, H young colleague of Przibram also began a similar investigation. He miniaturized the entire experiment to fit on the stage of an ultra-microscoDe and for a cloud substituted colloidal particles of metal produced in an electric arc. He could watch a single article as it floated between his plates (8). In principle this kperiment should haw given h i i excellent v&vs of P and made Ehrrnhaft the heru oithe srienr~ficu,orld. Hut in fact, Hrownian m~ltion,that erratic I~uzzinghehavior o i mulecult.~ colliding umith his cullo~dalparticles complrtely obliterated the effect drhnree. Thus Ehrenhaf't. relvina on the results of Przibram and his'own faulty data drew the c&clusion that the charee t its size (9-11 ). .. of a bodv was d e ~ e n d e non In 1910 pt:rslstence and determination began to pay off'fur Millikan. While making mensurrments on his diffusr rlo~lds, ~~
~
~
~
he noticed for the first time that small microscopic droplets of vapor seemed to hang balanced in the field "like stars aeainst a black backaround" (12). Bv changing the voltage on hll..i plates Millikon k u l d make these d r u ; ~ l e ~rlse s or at will. As hlillikan watched a druldrt hr cwlld see i t suddenly change its velocity as it gained acharged ion from the air, and could "count the electrons on them as easily as you can count the fingers on your hand." By sheer good fortune, Millikan's droulets were hundreds of times larger than the colloidal of Ehrenhaft and so were immune to the effects of Brownian motion. By 1911 he had designed a new apparatus usine droolets from a smav of hiehlv refined motor oil. He couli now obtain the charge on tlhe k~ectrouto better than 0.1% and show conclusivelv that electron charae - came in disCrete bundles (13-15). Meanwhile. Ehrenhaft (16) and two of his students, F. Zerner (17), and D. Konstantinowsky (18) were publishing numerous articles contending that Millikan's results were misleading because his drops were so large and uniform. Such ereat scientists as Born, Planck, and Sommerfeld listened in cunsternatim as IKhrrnhnft debated his results at a reaharch seminar 19,. Millikan, the upstart from the IJ S. was repurling what they knew to be correct yet they could not dissuade Ehrenhaft from his position that electron charges came in all sizes. ~ ~ Millikan's frustration was that no one seemed to recognize the imnortance of his work. He was either criticized or totallv ignored (19,. It was moit cl~*turhingr*, haw his own apparatus referred to as thc "Ehrenhaft-Millikan t~xperinientarrancement" (20,21). Very slowly the tide beg& to turn and the wave of criticism directed initially against Millikan hegan to take its toll on Ehrenhaft. By 1916, his enthusiasm dulled, Ehrenhaft no longer published on the electron charge. One must come to feel that by 1923, more than the need merely to honor Millikan, the awarding of the Nobel Prize was a trumpet blare announcing the final acceptance of atomicity as an established experimental fact. In his Nobel Prize address, Millikan allowed some of his years of frustration to surface (22) ~
.
After ten , w a r s of work in other laboratories in checkine the methods and rrsultc hdined III cmnmi
