IONS PRODUCED BY RONTGEN RAYS IN GASES AND VAPOURS. 257 



estimating the mobilities at pressures less than 1 cm. of mercury, the workable 

 voltages being necessarily small and the field thus being likely to be disturbed by the 

 presence of free ions. In general, however, voltages were chosen so as to render the 

 influence of the ionisation on the field practically negligible. 



B. Experimental Difficulties. 



Hysteresis. When the magnet circuit is broken, both weights W and W should 

 fall simultaneously ; if this occur, the point P in fig. 3 should correspond to a position 

 of K' on the movable scale, exactly on a level with the position of K. The position 

 of K' corresponding to a zero time interval between the breaking of the two contacts 

 could be accurately ascertained by the null method ; in general this position of K' 

 occurred when at the same level as K, but after the cells which worked the magnets 

 had been in use for a period, usually of about two weeks, it was ascertained that the 

 weights did not continue to fall simultaneously, an effect probably due to unequal 

 hysteresis in the two magnet cores. In such a case fresh cells were usually inserted, 

 but the effect gradually disappeared on reversing the current through the solenoids 

 of the magnet. 



Absorption of Charge by the Insulation. If the lower electrode B be at a fairly 

 high potential, e.g. 80 volts, and then the potential be reversed, a charge will be 

 present on the upper electrode A and its connections ; if now the lower electrode be 

 brought back to its original potential, there should on the whole be no free charge 

 on A ; with some vapours, however, it was found that a charge remained on A often 

 quite considerable in comparison with the charges due to the ionisation of the gas 

 when exposed to a flash of Rontgen rays. This residual charge was due to the 

 absorption by the insulation of a part of the charge resulting from induction. To 

 reduce this absorption to a minimum, use was made of the magnetic relay M (fig. 4). 

 As long as the weights W, W remained suspended, the arm D of the relay was in 

 contact with the point P, and so the lower electrode B was at zero potential ; but 

 when the magnet circuit was broken by the key F, the arm D came in contact with 

 the point Q while the weights were still falling freely, and thus the electrode B was 

 brought to the required potential. This potential was then reversed by the breaking 

 of the contact K, and finally B was brought back to zero potential. By thus restricting 

 the period during which B was charged to a very short duration, it was found that 

 the effect due to the absorption by the insulation of the charge resulting from induction 

 was negligible. 



5. Experimental Procedure. 



The two ionisation chambers were adjusted in such a way that the same flash from 

 the Rontgen-ray bulb produced in each equal quantities of electricity of any one sign. 

 Preliminary tests were made to ensure that the insulation was good and that there 

 was no appreciable absorption of the charge by the insulation. 



VOL. CCIX. A. 2 L 



