658 Mr. J. A. Crowther on the Secondary 



For the sake o£ clearness they are not shown in the first 

 diagram. 



The bulb used was a fairly large one of the ordinary focus 

 type, and was found to give a very accurate point-source of 

 X-rays. It was worked by a large Rudge coil, with a hammer 

 break, which was found to be capable, when used with a bulb 

 of moderate hardness, of maintaining a powerful and steady 

 stream of X-rays for a considerable period, without hardening 

 the bulb too rapidly. When properly adjusted it would run 

 the bulb continuously for ten or fifteen minutes without any 

 attention, and without appreciable alteration in the character 

 of the rays ; a property which rendered it very suitable for 

 the compensation method employed. 



The ionization in the ionization-chambers arises from three 

 sources : — 



(i.) The secondary radiation from the gas in the corre- 

 sponding gas-chamber, 

 (ii.) The tertiary radiation from the aluminium window d, 



and other parts of the apparatus, 

 (iii.) The spontaneous ionization of the air in the ionization- 

 chamber. 



It was necessary before proceeding with the actual experi- 

 ments to investigate each of these separately. 



Accordingly a Wilson electroscope was attached to each 

 of the ionization-chambers, and both the gas-chambers were 

 filled with air which had been carefully dried and freed from 

 all dust by passing through drying-tubes of calcium chloride 

 and phosphorus pentoxide, and finally through a long tube 

 filled with tightly pressed cotton-wool. One of the chambers, 

 A, was maintained at atmospheric pressure, to serve as the 

 standard ; the other, A! say, was exhausted, step by step, by 

 means of the water-pump. The rates of leak in the two 

 ionization-chambers for different pressures in A. 1 was measured 

 by comparing the rates of deflexion in the two electroscopes. 

 The spontaneous ionization in the ionization-chambers was 

 separately measured, by finding the rate of leak when the 

 X-rays were cut off, and was corrected for, in all the 

 readings. 



A curve could now be plotted showing the relation between 

 the pressure of the air in the chamber A, and the ratio of the 

 leak through the corresponding ionization-chamber B' to that 

 through the standard chamber B. The rate of leak through 

 B' when A' was completely exhausted was a measure of the 

 magnitude of the tertiary and other radiation from the 

 window and walls of the gas-chamber A x ; the remainder, 



