of Production of X Rays from a Coolidge Tube. 363 



with aluminium plate. The ionization was measured between 

 the aluminium plate A and the central plate B. The former 

 was connected to a 1000-volt battery and the latter to the 

 electrometer. Two additional plates C, D connected with 

 earth extended on both sides of the plate B. The cone of 

 rays entering the ionization vessel was fixed by the circular 

 opening 0, 2 cm. in diameter, in a lead plate, the size of 

 opening being adjusted so that no radiation fell on the 

 electrodes. The disturbing effect due to the X rays impinging 

 on the aluminium end of the box was small, and could be 

 neglected. In order to determine the absorption curve for 

 water, a vessel was constructed consisting of two closely 

 fitting brass tubes E and F. Openings, G, H, cut in the 

 ends of these tubes, were covered with a thin sheet of mica. 

 The length of the column of water in the path of the rays 

 could be simply adjusted, and care was taken that the 

 radiation entering the ionization vessel did not strike the brass 

 ends of the vessel. The maximum length of the column of 

 water in the experiment was 5 cm. It is seen from fig. 2 

 that this is sufficient to reduce the intensity of the radiation 

 to a small fraction of its initial value. 



Fi<r. 2. 



In order to determine the absorption of the end part of the 

 radiation by water, the vessel EF was removed, and a suf- 

 ficient thickness of aluminium introduced in the path of the 

 ravs to reduce the ionization to an equal degree. The water 

 column was again introduced, and the experiments continued 

 up to a thickness of 15 cm. of water. 



The capacity of the circuit and of the condensers in parallel 

 was carefully determined. Changes in the intensity of the 



