474 Mr Baker, Influence of Ultra Violet Radiation 



A D'Arsonval galvanometer was placed in circuit, but if there 

 was any change in the current when the rays fell on the kathode 

 it was certainly much less than 0'5 per Cent. This is not sur- 

 prising, as Kreussler's 1 greatest observed photo-electric current 

 from zinc was only of the order of 10 -8 amperes, while the current 

 through this tube was of the order of 10 -4 amperes. 



By shunting the current from part of the cells across a liquid 

 high resistance, and picking off the current at any required point, 

 it was possible gradually to vary the potential difference between 

 the electrodes in the tube. The electrostatic voltmeter connected 

 from one end of the shunt to the anode of the tube indicated 

 the change of potential. The following numbers give the total 

 potential difference across the tube, i.e. to the electrometer read- 

 ings are added the potential produced by the part of the battery 

 not shunted. Where several readings are given they indicate 

 different measurements. 



Radiation falling on kathode, 



Discharge starts 513, 513, 514. 



Discharge will not start 512, 512, 512. 



This discharge was very faint and could not be seen when the 

 spark gap was in action, but it continued after the sparks were 

 stopped. The electrometer reading fell three or four volts when 

 the discharge was passing but rose again as soon as it was 

 stopped. As no change in the electrometer reading could be 

 detected on starting the sparks, when the potential across the tube 

 was 512 volts, it was concluded that no measurable discharge 

 passed while the light was on the kathode, and certainly the tube 

 remained perfectly dark after the sparks had been stopped. 



Next was measured the lowest potential that would start the 

 discharge in the dark, and this was found to be 557, 558, 558. 

 A thin glass plate was next put in front of the window and the 

 discharge could not be got to start by means of the rays at 

 554 volts, but out of six or eight trials it went once at 156 and 

 once at 155. 



Coming back to a repetition of the first measurements, it 

 was found possible to start the discharge by means of the ultra 

 violet light at 511, 511, 510 volts, showing that the gas was 

 gradually becoming weaker with regard to the potential required 

 to break it down. In the above cases the discharge would not 

 start at 509, 508 volts. 



Next, the discharge was started, the rays were cut off, and 

 then the potential was gradually reduced until the discharge 

 ceased. The readings of the electrometer, taken immediately 



1 Ann. der Physik, Bd. v. no. 10, 1901. 



