NATURE 



41 



THURSDAY, MARCH 21, 1918. 



tTUE ATOM OF ELECTRICITY, 

 he Electron: Its Isolation and Measurement and 

 the Determination of Some of its Properties. 

 By Prof. R. A. Millikan. (University of 

 Chiciig-o Science Series.) Pp. xii + 268. 

 {Chicago, 111.: University of Chicago Press; 

 London: Cambridge University Press, 1917-) 

 Price 1.50 dollars net. 



THE great advances in physical science that 

 have been made during the past twenty 

 years have been largely based on the idea that elec- 

 tricity, like matter, is not infinitely divisible, but 

 that there exists a definite fundamental unit of 

 electrical charge or "atom of electricity" which 

 is incapable of further subdivision, and that all 

 charges, however great, are integral multiples of 

 this unit. While the great mass of experimental 

 observation strongly supported this idea, it has 

 been extremely difficult, as in the case of so many 

 fundamental theories, to obtain a direct and con- 

 vincing proof of its truth. 



In the present monograph Prof. Millikan, of 

 the University of Chicago, gives a most interest- 

 ing and complete account not only of the general 

 history of this idea, but also of the methods that 

 have been developed to determine, with an 

 accuracy, it is believed, of one in a thousand, the 

 magnitude of the fundamental unit of charge or 

 atom of electricity. When we consider that the 

 atom of electricity is probably the most funda- 

 mental and important physical constant in Nature, 

 it is a great triumph for experimental science to 

 have devised within so short a time methods 

 capable of such accuracy. It is to Prof. Millikan 

 himself that we are indebted for the final successful 

 methods of attack of this most difficult problem, 

 and in this monograph we have the advantage of 

 obtaining a first-hand account of his difficulties 

 and triumphs. 

 ; From the title of the book, "The Electron," it 

 j might be inferred that the author dealt with the 

 i general properties of the electron and its rdle in 

 ' electric phenomena. This, however, is not the 

 r.ise, for the first half of the book contains an 

 ;ucount of experiments to prove the existence of 

 an atom of electricity and to measure its value, 

 and the latter half is devoted to problems of atomic 

 structure and radiation. There is, however, some 

 historical justification for the title, since the late 

 Johnstone Stoney, when he first introduced the 

 •name "electron" in 1891, restricted its meaning 

 to the actual magnitude of the unit charge, quite 

 ^part from the mass or properties of the carrier 

 itself, which at that time were quite unknown. 

 W'hile the original meaning has to some extent 

 ■been conserved, there is now a generaL tendency 

 to restrict the term "electron " to those atoms of 

 'disembodied ni^gative electricity like the cathode 

 l>articles and $ particles of radium which have an 

 -apparerit mass srn&ll compared with that of the 

 hydrogen atom. When the smalliest mass asso- 

 NO. 2525, VOL. lOl] 



ciated with the atom of positive electricity has 

 been fixed, it would naturally be termed the "posi- 

 tive electron." So far, however, there appears to 

 be a fundamental distinction between positive and 

 negative electricity, for the atom of positive elec- 

 tricity has never been found associated with a 

 mass less than that of the hydrogen atom. 

 Whether the nucleus of the hydrogen atom is in 

 reality the positive electron or whether it may 

 prove a complex is still an unsettled question. 



After a simple sketch of the history of the 

 subject, the author passes in careful review the 

 pioneer methods of J. S. Townsend, Sir J. J. 

 Thomson, and H. A. Wilson for measuring the 

 unit of charge carried by electrified drops of water, 

 and points out the inherent difficulties of accurate 

 measurement under these conditions. The methods 

 employed by Prof. Millikan were similar in general 

 principle to those used in these early researches, 

 with one important distinction. Instead of measur- 

 ing the average charge carried by a multitude of 

 water drops, subject to variation of size by con- 

 densation or volatilisation, he confined his atten-^ 

 tion to a single charged drop of oil or mercury of 

 small diameter. 



A sprayer was used to produce fine drops of 

 oil or mercury, and some of these, charged by 

 friction, fell through a small opening into the 

 space between two charged parallel and horizontal 

 plates, and came into view of a microscope as 

 bright points of light. By adjustment of the elec- 

 tric field a single drop of this kind could be held 

 suspended in the field of view of the microscope 

 for hours at a time. The charge on the drop 

 coukl be determined from observations on the 

 electric field required to balance the drop, and the 

 velocity of fall of the drop under gravity. The 

 drop usually carried a number of unit charges 

 either positive or negative. By an ingenious 

 method he was able to vary and even reverse this 

 charge at will. For this purpose the drop was 

 allowed to pass for a short time into a region 

 where there was an excess of ions of one sign 

 produced by radium rays and separated by an 

 electric field. In this way it was possible to un^ 

 load the charge of the drop unit by unit, and to 

 measure it after each exposure. By direct ex- 

 periment of this kind the author was able to show 

 definitely that all charges gained or lost by the 

 drop were either single or integral values of a 

 definite unit, and still more that the original charge 

 produced on the drop by friction was an integral 

 number of this unit. In this way he was able to 

 prove a number of fundamental points in a most 

 direct way ; Tor example, that the unit of charge 

 produced by friction is the same as the charge 

 carried by the ion in gases, while the unit of posi- 

 tive charge was shown to be identical with the 

 unit of negative charge within a very small margin 

 of error. 



As a result of five years' work involving a com- 

 plete study of numerous corrections, the value e 

 of the atom of electricity was found to be 

 c = 4'774x lo-i" electrostatic units, with a probable 

 error of not rhore than one in a thousand. With 



