218 Mr Townsend, Secondary Rontgen Rays. 



different bodies, and the intensity of the secondary radiation 

 compared with that of the primary radiation which excites it. It 

 is arranged in a manner suitable for examining the intensity of 

 the secondary radiation at a distance of some centimetres from 

 the radiating body. 



The Crookes tube B, and the coil with which it was worked, 

 were contained inside a box covered with lead having an aperture 

 (4 centimetres square) at A. A lead pipe of square section was 

 placed over the hole, A, so as to confine the rays from the bulb to 

 a small portion of the space above the box. This space was 

 explored by means of the apparatus E, which consisted of a gauze 

 cylinder G, and an axial electrode G, fixed to a moveable piece of 

 wood W, by means of insulating supports P and S. The cylinder 

 was connected to one terminal of a battery of lead cells, the other 

 terminal being to earth. The electrode was connected to the 

 insulated quadrants of an electrometer. 



When the rays are traversing the space outside C none of the 

 ions which are produced can arrive at the electrode G ; but when 

 rays fall on G some traverse the gauze and produce ions inside the 

 cylinder. The deflection of the electrometer in a given time is 

 proportional to the number of ions which are produced inside C, 

 if the potential of G is sufficiently high to produce the maximum 

 deflection. In the present experiments the potential of G was 

 85 volts ; when double that potential was used, the deflections 

 were not increased by 2 per cent., so that the difference of potential 

 of 85 volts suffices to collect practically all the ions of one sign 

 produced inside G on the electrode G. 



The apparatus E was placed in the position shown in the 

 figure, so that no direct rays from the bulb could fall on it. A 

 series of experiments were then made with different substances 

 placed above the bulb at D. Screens, connected to earth, were 

 put up round the wire F to prevent any ions that are produced in 

 the space outside G from being collected on F. In order to avoid 

 complicating the figure these screens are omitted. 



When a heavy metal is placed at D, and the rays from a 

 Crookes tube allowed to fall on it, a large deflection is obtained on 

 the electrometer scale, showing that there are rays given off from 

 D which ionize the gas inside G. When there is no radiator at D, 

 except air, the electrometer gives a small deflection when the rays 

 are emerging from the bulb. This small effect is probably due to 

 the secondary rays given out by the air. 



The following table gives the deflections obtained in 10 seconds 

 with various substances at D. The first column gives the figures 

 obtained when the rays from D arrived at G without passing 

 through any other material than the air. The second column 

 gives similar numbers obtained by cutting down the rays from 



