276 



On the X-ray Spectrum of Nickel. 



position of the cathode spot on the anti-cathode. To eliminate 

 the time factor two electroscopes were used. One of these 

 was mounted on an ionization chamber as shown in figure 2. 



Fiff. 2. 



The chamber consisted of a framework of brass 6 cm. square, 

 covered with exceedingly thin aluminium foil. The leaf 

 support projected downwards through a sulphur plug into 

 this chamber. The aluminium side was placed close in front 

 of the window of the X-ray bulb so that all the radiation 

 emerging passed through the ionization chamber. In order 

 to plot the absorption curves of the X rays, a second electro- 

 scope was used w r hich had one side composed of fine aluminium 

 foil. This was placed some distance behind the ionization 

 chamber, but also so as to receive all the radiation cornino- 

 from the window. The ratio of the number of divisions 

 passed over by the leaf of the second electroscope to those 

 passed over by the leaf of the first electroscope during the 

 time the discharge was running, was taken as a measure of 

 the intensity of the ionization in the second electroscope. 

 The absorption curves were found by placing thin sheets of 

 aluminium foil in front of the aluminium side of the second 

 electroscope. The hardness of the bulb was measured by the 

 length of the alternative spark-gap. 



With the tube at 30,000 volts it was found that 79 per cent, 

 of the radiation was absorbed exponentially with a value of 



- = 55*5 in aluminium. This is obviously the characteristic 



K radiation of nickel, which Barkla * found to have a mass 

 absorption coefficient 59*1. The remaining 21 per cent, 

 was absorbed approximately exponentially with a value of 



* Phil. Mag. vol. xvii., May 1909. 



