240 Mr Kaye , The Selective Absorption of Rontgen Rays. 
that of a kind closely resembling the primary rays. It was, of 
course, essential to compare incident radiations by their direct 
ionizing effect, complicated as little as possible by any secondary 
radiation. 
The electrical connections are shown in figure 2. 
The outside of the ionization chamber was raised to a potential 
(200 volts) sufficient to give the saturation current for all the 
degrees of hardness of the Rontgen rays measured. The potential 
was supplied from a battery of small accumulators, with the nega- 
tive pole earthed. 
A Wilson tilted electroscope E was used to measure the leaks, 
and with its great range of sensitiveness, obtainable without the 
aid of any added capacity, proved extremely convenient for the 
purpose. If the electroscope was adjusted for great sensitiveness, 
a potential divider P, giving a control of ^ volt, was used as a fine 
adjustment on the potential of the charged plate, which ordinarily 
was raised to something in the neighbourhood of 200 volts. 
Indeed this potential divider proved to be a real convenience 
for all degrees of sensitiveness, and the zero of the electroscope, 
taken when the entire gold leaf system was earthed, could be kept 
perfectly constant. The movement of the gold leaf over a definite 
range was observed through a small telescope having a scale in 
the eye-piece. The times were taken with a stop watch. 
A calcium chloride solution key K, which was operated from a 
distance, made or broke the earth connection. It is perhaps worth 
while laying stress on the advantages that a solution of metallic 
salt has over mercury for a key. The kick so common with mercury 
keys on making or breaking connection is quite prevented. The 
electrification produced by the splashing of solutions of salts is 
very small ; for mercury it is large. 
