lln //. .1. Buimtead — Emission of Electrons by 



inl point is the mean of from three to six separate observa- 

 tions, taken in various orders and with frequent returns to the 

 measurement of the current with no foils, to check the con- 

 stancy of the electroscope readings. Several repetitions of 

 these measurements gave entirely similar results. 



Before entering the case the a-rays passed through 06 cin of 

 air and through the thick aluminium foil which covered the 

 holes in the top and whose air-equivalent was l*47 cra , so that, 

 with this apparatus, only the upper part of the polonium curve 



Fig. 2. 



10 













U 





*^ 



^ 







p 







r 







4 







i 















__^i 



? 























.01 



■03 04 



VOLTS PER MINUTE 



.05 



. Curve I, Polonium ; Curve II, Thorium Active Deposit. 



can be obtained. To obtain a-rays of greater range, I used 

 the active deposit from the thorium emanation, which emits 

 a-rays of two different speeds ; the rays from thorium B have 

 a range in air of 5"0 cm , and those from thorium 0, of 8 - 6 cm . 

 An additional advantage is the slow rate of decay of the thorium 

 active deposit, which falls to half-value in 10 - 6 hours. 



A small quantity ("activity of 24 mg Ra.Br, ") of Professor 

 Hahn's mesothorium, obtained from Knofler & Co. of Berlin, 

 was available. This was placed in a small cylindrical cup of 

 platinum with a hemispherical bottom, and was slightly mois- 

 tened to increase its emanating power. The top of the cup 



