344 Sir E. Rutherford, Prof. Barnes, and Mr. Richardson : 



As it was very important to keep the voltage constant 

 during an absorption experiment, it was necessary to control 

 the voltage within small limits by means of an adjustable 

 point discharger D placed near one of the high potential 

 conductors. The arrangement for this purpose is seen 

 clearly in fig. 1. The voltage galvanometer worked through- 

 out in a very satisfactory manner, and it was possible by its 

 aid to keep the potential steady within about one per cent, 

 over the interval of an hour or more required for a complete 

 absorption experiment. 



Since the galvanometer was highly damped, it would not 

 indicate any rapid surges in the voltage. These surges, 

 however, occasionally made themselves manifest at the 

 extreme end of the absorption curve, where the intensity of 

 the radiation had been reduced to about 1/1000 of its initial 

 value. Even when the voltage appeared quite steady by the 

 galvanometer, the presence of surges could be detected by 

 the irregular rate of movement of the electrometer. 



Measurement of Current, 



A moving-coil galvanometer E was placed in the main 

 circuit to measure directly the current delivered by the 

 machine, and was protected against electrostatic disturbances 

 by a metal shield FF, as in the other case. The actual 

 current passing through the bulb was measured in the 

 following way. The deflexion was first observed under 

 the experimental conditions of excitation of the radiation. 

 The heating circuit was broken, and the voltage retained at 

 the same value by means of the point discharge and by 

 altering the speed of the machine. The deflexion rapidly 

 dropped to a constant value, which was due mainly to the 

 current taken by the voltage galvanometer, but partly also 

 to conduction over the surface of the heated bulb. The 

 difference between these two readings thus gave a definite 

 measure of the current passing through the bulb, quite 

 independently of all other losses in the circuits. The current 

 passing through the bulb in the various experiments at 

 different voltages varied from 1/100 to 6/100 of a milli- 

 ampere. It was found in all experiments that the ionization 

 for a given voltage applied to the tube was directly pro- 

 portional to the current through the bulb — in other words, 

 the intensity of the radiation was directly proportional to 

 the number of electrons incident on the anticathode. 



