hy Homogeneous Rontgen Radiations. 



417 



(for a given radiator), no discordance in the results can arise from 

 variations in the bulb. 



The electrode D v^as parallel to L and distant 1 cm. from it. 

 D was connected to the gold leaf of the secondary electroscope, 

 and could be earthed by a key. The chamber A was kept at a 

 potential of 200 volts. 



When the bulb was in action the air between D and L was 

 ionised. Thus a charge was given to D, as soon as the connection 

 to the earth was broken. The method of experimenting was to 

 vary the pressure of the air inside A, and to measure the ionisa- 

 tion between D and L corresponding to each pressure. The 

 amount of this ionisation was in each case standardised by means 

 of the primary electroscope. 



TO "PumP 



^ TO SeCQN'DAKi 



BLECTROSCOVe 



"P«Tch.Tn 



SM. 



Pr irnwryTXect r 04 c o|) e 



Fig. 1. 



This ionisation is due to two causes: (1) Ionisation due to 

 X-radiation alone. This has been shown by Crowther* to vary 

 directly as the pressure of the air. (2) Ionisation due to cathode 

 particles emerging from L. The amount of ionisation due to this 

 source will remain constant as long as the pressure is great enough 

 to absorb all the particles. When the pressure falls below a certain 

 value some of the particles will reach D before being absorbed and 

 the ionisation will decrease. 



Fig. 2 shows how the ionisation due to each of these sources 

 varies with the pressure, and how the actual curve found is the 

 sum of these separate effects. 



Crowther, Proc. Roy. Soc, Ser. A, lxxxii., March 10, 1909, pp. 103—127. 



