Mr Townsend, Secondary Rontgen Rays. 221 



the metal plate A, which was thick enough to prevent any rays 

 passing through it. 



(It was found that brass, zinc, or copper plates two millimetres 

 thick prevented any radiation from passing through, but it would 

 have required a much greater thickness of aluminium to produce 

 a similar effect, so that the aluminium used at A was backed 

 with a brass plate. The aluminium plate was two millimetres 

 thick, which was more than sufficient to stop the secondary 

 radiation from the brass from extending to the air between A 

 and B.) 



The plate A was moveable and insulated, being connected by 

 means of an ebonite junction to a bar V, which moved vertically. 

 The position of A could therefore be easily varied, and its distance 

 X from B was obtained from the micrometer screw which fixed 

 the position of V. 



The plate A was connected to the insulated quadrants of an 

 electrometer which gave a deflection of 90 scale divisions for a 

 potential difference of 143 volts between the two pairs of quad- 

 rants. The plate B was connected to one terminal of a battery of 

 lead cells, the other terminal being connected to earth. The 

 number of cells used was proportional to the distance between the 

 plat .3. It was found that 80 cells per centimetre was sufficient; 

 when the force was doubled the electrometer deflections were 

 increased by only one or two per cent. 



A series of experiments were made with the plates at various 

 distances apart and the following table gives the electrometer 

 deflections, S, when the rays ionized the gas between the plates 

 for 15 seconds. The observations were made several times and the 

 means taken so as to eliminate errors arising from variations in 

 the strength of the rays. The distances X are given in milli- 

 metres. 



The different columns contain the results of observations made 

 with different metals at A. The same aluminium plate B was used 

 in all cases. It was impossible to get exactly the same strength 

 of rays when using different metals so that the deflections obtained 

 with zinc, copper, and aluminium, were multiplied by factors so as 

 to make the difference between the observations at 10 and 15 centi- 

 metres to be, in each case, 18, which was the difference obtained 

 when a brass plate was used at A. 



The capacity of the condenser which is connected to the 

 insulated quadrants varies with the distance between the plates. 

 In order to obtain numbers proportional to the charges acquired 

 by the plate A the above values of 8 (which are potentials in 

 arbitrary units) must be multiplied by the capacities correspond- 

 ing to the different values of X. 



The following are the capacities in centimetres of the conductor 



VOL. X. PT. IV. 16 



