406 Mr. H. Lester Cooke on Penetrating 



be determined by observing the rate of collapse of the leaves 

 when the rod which supported the bead was earthed, and then 

 repeating- the observation with this rod at a potential of 300 

 volts. The difference gave the leakage across the bead for a 

 P.D. of 300 volts. This was generally found to be about 

 •02 division per hour, and was negligible in comparison 

 with the leakage due to the ionized air. After charging the 

 leaves the rod C w^as always earthed. This had the effect of 

 slightly reducing the potential of the insulated system. This 

 lowering of the potential, which could be accurately measured, 

 did not in any way interfere with the results, as it was constant 

 throughout the experiments. 



The voltages employed ensured the removal of practically 

 all the ions^ the potential gradient across the gas, according 

 to the measurements of the instrument, never falling below 

 20 volts per cm. 



The primary object of this research was to ascertain 

 whether there was any penetrating radiation coming from 

 some external source, and responsible for the ionization of 

 the air inclosed in the electroscope. For this purpose slabs 

 of lead were cast, an inch in thickness, which could be built 

 in the form of a cage for the insertion of the instrument. 

 Ijead w^as chosen as the material for these screens on account 

 of its high absorptive power for ionizing radiations, as com- 

 pared with its density ; this ratio in the case of lead being 

 nearly double the value of the same ratio in the case of most 

 other materials. 



The instrument was charged, then placed in a definite 

 position on an earthed support, w^hich was carefully levelled, 

 and the deflexion of the leaf observed. The instrument was 

 then placed inside the lead cage and allowed to remain for 

 about twelve hours, then removed, set up as before, and the 

 deflexion ao-ain read. Great care was taken during these 

 operations not to jar the instrument m any way. it was 

 found that moving the instrument in this way did not intro- 

 duce an error of more than '2 of a division ; and as the 

 observed collapse of the leaves was usually between 20 and 

 40 divisions this did not introduce any serious source of 

 inaccuracy. 



The effect of surrounding the electroscope with lead screens 

 was always to reduce the rate of discharge of the leaves. 

 The inch slabs of lead used in the first experiments diminished 

 the ionization as much as 25 per cent. This result w-as 

 arrived at by a continued series of observations extending 

 over more than a month, the readings being taken and the 

 instrument recharged night and morning. 



