Spectra of Helium, fyc. in the Ultra-Violet. 295 



In this formula v is the threshold frequency for the 

 copper target, that is to say the frequency at which photo- 

 electric emission begins from copper. For this we take a 

 weighted mean of the values determined by Richardson and 

 Oompton *. This mean is '38 x 10 15 . We have therefore, 

 finally, 



Putting; in the values of r „ and » . for each magnetic 



o max. mni, o 



field, we get directly the values of v max and z^ min These 

 values are tabulated below. Fig. 4 is a graphical represen- 

 tation of this table. The value of the threshold frequency 

 exerts a very large effect on the determinations at low 

 frequencies, but at high frequencies the proportional effect 

 is of course much less. 



B. 



''max. 



^max. 



v min. 



X min. 



4 



1-096 



2730 



1-04 



2880 



8 



1-445 



2069 



1-224 



2450 



12 



2-024 



1485 



1-529 



1960 



16 



2-836 



1057 



1-977 



1515 



20 



3-880 



773 



2-512 



1195 



24 



5-155 



582 



3-166 



948 



28 



6-663 



450 



3-964 



757 



32 



8-403 



357 



4-910 



611 



36 



10-35 



289 



5-900 



508 



40 



12-578 



238 



7-060 



424 



44 



15-02 



199 



8-358 



358 



48 



17-68 



170 



9-774 



307 



If now the radiation is not a continuous spectrum but 

 consists of lines, we might expect to locate the individual 

 lines directly with greater or less definition depending on 

 the width of the slits, but, as a matter of fact, the known 

 character of the velocity distribution amongst the photo- 

 electrons liberated by monochromatic light renders this 

 impossible. This distribution as determined by Richardson 

 and Compton has the form shown in the curves at the 

 bottom of fig. 5. For purposes of illustration the total effect 

 obtained from seven lines of equal intensity falling simul- 

 taneously on the target can be determined by summing up 

 the seven corresponding curves as shown in this figure. If 

 we take a series of observations of the electrometer deflexions 

 corresponding to a series of values of the magnetic field from 

 zero field up to the field necessary to give zero deflexion of 

 the electrometer, we get, therefore, a curve like the resultant 



* Phil. Mag., Oct. 1912. 



