40 Journal of the ^Mitchell Society [September 



presided over formerly by him and now by his former pupil, Sir 

 Ernest Rutherford. Among experiments performed by Thomson 

 was one on the canal or positive rays, to which we shall have occasion 

 to refer later. We may note in passing, however, that while for 

 cathode rays whose velocity does not approach that of light he found 



-^ equal to 1.7 x 10''. for the positive rays the greatest value ob- 

 m ^ * . 



served was 10^, Avhit'h is also the value for the hydrogen ions in elec- 

 trolysis, thus reenforeing conclusions as to the relative size of the 

 electron. 



A series of experiments was performed in the Cavendish labora- 

 tory, both by Thomson and his pupils, to determine the value of e, 

 the charge carried by the electron. The value of this most im- 

 portant physical constant has been determined by Millikan in a 

 series of admirably planned and accurately executed experiments 

 which extended over several years. His method was an important 

 modification of one of the Cavendish laboratory experiments. His 

 findings were very concordant, and are accurate to 0.07 of 1 per 

 cent ; the value obtained by him is now the generally accepted one. 



Rutherford and Geiger devised an electrical method bj^ which 

 they could count the number of alpha (a) particles emitted per sec- 

 ond by radium. By measuring the charge on the counted number 

 they found that each alpha (a) particle carried two positive elec- 

 tronic units of charge, a finding in harmony with Rutherford's spec- 

 troscopic work, which showed that alpha (a) particles were helium 

 ions. Making use of the rate of emission of alpha (a) particles by 

 radium, Rutherford in 1911, with the aid of Geiger and Marsden, 

 performed another important experiment. He computed the chance 

 that an alpha (a) particle in being shot through thin sheets of 

 gold and other metal foils would suffer a given deflection because 

 of the positive nuclei of these metals. ]\Iost of the alpha (a) par- 

 ticles in passing through the foils move in .straight lines but some are 

 deviated. Rutherford's theory was that whenever an alpha (a) par- 

 ticle passes sufficiently near the nucleus it will be deflected bj'- the 

 charge of the nucleus and be deviated. The experiment involved 

 finding what fraction of the alpha (a) particles, which were shot 

 through the foils, produced scintillation on a screen at a position 

 which corresponded to the comjiuted angle of deflection. From cal- 

 culations based on this experiment he concluded that the number of 



