﻿Ranges of tlie a Particles from Uranium. 



141 



10 cm. diameter, were drilled through the brass plate CO. 

 An exactly similar but much thinner brass plate DD could 

 be screwed on to the lower side of the plate CC. Between 



Fiff. 1, 



illtillllllll 1111 



these two plates a thin and uniform sheet of mica of stopping- 

 power equivalent to 8 mm. of air was placed and tixed down 

 air-tight. On account of its position between the metal 

 grids the mica was secured against breakage, and w T as then 

 easily able to withstand an excess of 20 cm. pressure from 

 either side ; this difference of pressure was frequently 

 necessary in the course of the experiments. The lower bell- 

 jar BB contained the active film FF at a distance of 9*8 cm. 

 from the mica. On account of the metal grid only the 

 a. particles emitted at angles less than 20 degrees to the normal 

 could pass through the mica into the upper bell-jar which 

 contained the electroscope. The latter w T as cylindrical in 

 shape and was 8 cm. high and 10'5 cm. in diameter. It 

 could be charged from outside through the wire R which 

 could be rotated by means of" the ground-glass joint T. 



In the experiments the upper bell-jar was first completely 

 exhausted and then filled with hydrogen up to a pressure of 

 about 9 cm. of mercury. The use of hydrogen is preferable 

 to air since, according to Taylor * and others, the rise of the 

 ionization curve is more pronounced in the former gas than 



* T. S. Taylor, Phil. Mag. vol. xviii. p. 604 (1000) : vol. xxt. p. 571 

 (1011). 



