130 Dr. J. Robinson on the Photoelectric 



the molecule, and thus V = 0. V is the energy that an 

 electron must acquire in order just to emerge from a mole- 

 cule. Hence, for oxygen, Y = kn, where n is the frequency 

 corresponding to X 1350. 



He did not investigate the law for platinum, but he showed 

 that it holds rigorously for a large number of metals and also 

 that k does not vary much from metal to metal. We will 

 take k for platinum to be the mean for the different metals 

 investigated, i. e. k = 3*62 x 10 ~ 15 , and assume that the 

 longest wave-length which is capable of producing a photo- 

 electric effect in platinum is \ = 3400_, which was near the 

 wave-length limit for cadmium. 



This gives V =3*25 volts, which gives the minimum 

 energy to produce a photoelectron in platinum. If there 

 are electrons in the metal with velocities larger than this, 

 then it will be possible to produce secondary electrons by 

 collision. By the application of the law Y = kn — V we can 

 easily find whether there are electrons present with velocities 

 greater than 3' 25 volts. By assuming a high enough value 

 for n, and this is only limited by the temperature of the 

 lamp and the absorption of quartz, electrons can be found 

 with velocities greater than 3*25 volts. This limitation of 

 quartz on the frequency may prove a drawback. For 

 instance, if X 1849 is the shortest wave-length which falls 

 on the film, we find V to be 2*65 volts, which is less than the 

 minimum to produce ionization. [This theoretical value for 

 the maximum velocity agrees fairly well with the actual 

 values observed, see §4.] 



The discovery of Dyke enables us to get over this diffi- 

 culty. For thin films the maximum velocity is much larger 

 than for thick films, and Hughes's results only apply to thick 

 films. If we take a moderate estimate of the ratio of the 

 velocities for thin films to those for thick films as 3, from 

 Dyke's results, there will be electrons in ordinary thick 

 metals with velocities of 2*65x3 volts, which is well 

 outside the limit of 3*25 volts required for ionization to be 

 possible. 



12. The decrease in magnitude of the photo-currents 

 following after the sudden increase can possibly be ex- 

 plained by an absorption of the electrons. The incident 

 current does not decrease as much as the emergent, for the 



F 



ratio j- diminishes. If the experiments had been extended 



to much thicker films, possibly the incident current would 

 have been found to increase again ; for Ladenburg showed 



