emitted by an Electrical Discharge at Low Pressures. 415 



the current, but it is doubtful i£ this is so for the lower 

 pressures. The current in all the following results was 

 always 4 milliamperes. 



This current was obtained partly by varying the number 

 of cells in series with the discharge-tube, and partly by 

 altering the resistance R. At the higher pressures the dis- 

 charge usually required to be started by connecting one only 

 of the terminals to one of the poles of an induction-coil for 

 an instant ; but at the lower pressures the discharge practi- 

 cally always started itself. The lead box being connected to 

 earth was useful in shielding the detecting apparatus from 

 disturbances due to the coil and other sources ; the coil (not 

 shown) being in the box. 



The distribution of intensity was obtained as the mean of 

 several sets of readings for several different pressures, and 

 the whole set were then connected by fixing the tube at a 

 position which gave a large intensity in most of these cases, 

 reproducing these different pressures, and getting the corre- 

 sponding deflexions. The intensities could thus be all ex- 

 pressed in terms of some arbitrary unit. 



Plottincf the Results. 



The distance between the electrodes being 1\ cm., a dis- 

 tance of 1\ units was marked off along the axis of #, the 

 anode being at the origin and the cathode at the point 7*5. 

 The intensity of the light from the first cm. of tube beginning 

 at the anode was taken as ordinates at pt^ = *5, from the 

 second cm. at # = 1*5 and so on, and from the last cm. which 

 overlapped the previous one at # = 7. By this means the 

 curves figs 2-6 were obtained (pp. 416-418). 



The current as already stated being always 4 milliamperes, 

 the pressure being given in millimetres of mercury, and the 

 potential difference V between the electrodes when current 

 was passing in volts. 



Discussion of He suits. 



The anode is in all the figures at the origin and the cathode 

 to the right, and it will be seen that for all the pressures 

 considered the emission of ultra-violet light from the half 

 centimetre beginning at the cathode is practically negligible. 

 The very large cathode fall of potential (about 340 volts) 

 does not extend beyond this space, and the results therefore 

 confirm the theory that practically all the energy in this 

 space is devoted to forming fresh ions by collisions, and very 

 little of the energy is available for emitting radiations. 



