124 



Dr. J. Robinson on the Photoelectric 



detected most quickly by comparing the effects due to the 

 light from a spark and from a mercury arc *. 



A spark was obtained between brass terminals, and was 

 excited by an induction-coil through the primary of which 

 an alternating current was sent. The capacity in parallel 

 with the spark-gap was varied, and Millikan's results verified 

 that in this way the photo velocities could be altered con- 

 siderably. The spark-gap was arranged between the mercury 

 lamp and the photoelectric cell so that the photo-current 

 from each could be measured at will. The intensity of light 

 from the spark was also very constant so that no corrections 

 for variation in intensity were necessary. A liquid-air 

 vacuum was not obtained, but the pressure was kept as low 

 as possible by a Gaede pump. The orders of the velocities 

 obtained were 1-2 volts from the arc, and 50 volts from the 

 spark. 



A whole series of films was investigated as in the preceding 

 sections, and the photo-currents (incident and emergent) 

 measured for each source of light. In Table IV. the actual 



Table IV. 





Arc. 





Spark. 





Thickness 

 of 



















Film. 



Currents. 



E 



Currents. 



E 





E. ] I. 



I ' 



E. 



I. 



I * 



20 seconds . . . 



3 2 



1-16 



2 



1-5 



1-27 



40 „ ... 



4 



... 





2 







60 „ ... 



37 



32 



1-16 



32 



27 



1-20 



80 „ ... 



560 



480 



1-17 



185 



148 



1-23 



110 „ ... 



440 



430 



1-024 



12S 



119 



1-075 



140 r , ... 



250 



310 



0-79 



84 



99 



0-85 



i . ... 



Velocities. 



E 



Velocities. 



E 





E. 



I. 



I * 



E. 



I. 



I ' 



140 seconds ... 



1-5 Volt. 



1-8 



083 



51 Volts. 



54 



094 



magnitudes of the currents are given in scale-divisions of the 

 electrometer, as well as the ratios y currents. The results 



* In making 1 measurements on this point, I was assisted by Mr. J. W. 

 Buckley, of Sheffield University, to whom I express my best thanks. 



