IONS PRODUCED BY RONTGEN RAYS IN GASES AND VAPOURS. 253 



charge added to the plates A and A', but as soon as there is any appreciable interval 

 between the Rontgen ray discharge and a succeeding reversal of potential on the 

 plate B, a charge of electricity is obtained which can be detected by the ordinary 

 electrical means, e.g. by connecting the two plates to an electrometer or electroscope. 

 In this way the position of the nick at P or of the corresponding point O on the time 

 axis can be determined. Tn a similar manner, in order to determine the position of 

 the point Q, we would connect the plate A to a plate A', adjusted in such a way as 

 to continually receive a charge represented by the straight line FGH in the figure. 

 Thus the two plates jointly would receive a charge KST, and the position of S or Q 

 could be readily determined by noting the time interval t, on one side of which we 

 obtained a charge on the measuring instrument and on the other side of which no 

 charge was obtained. 



This null method of obtaining the positions of the nicks in the different curves was 

 employed in most of the experiments described in this paper. In actual practice the 

 charge given to the auxiliary plate was adjusted in such a way that on one side of 

 the nick whose position was sought we obtained a very small constant charge (instead 

 of no charge as in the theory), while on the other side^ve would obtain charges of a 

 sign opposite to this small charge. By using this method the nicks were well defined 

 and thus a high degi-ee of accuracy was obtainable. In addition, it can be readily 

 seen that the null method is independent of the intensity of the flash of Rontgen 

 rays, whereas, if the complete curve be drawn as in the figure, the flash has to be 

 maintained constant in intensity throughout the observations. It is worthy of 

 mention that, under the conditions just described, the mobilities are really deter- 

 mined for those ions which are formed in the thin layer of gas or vapour in the 

 vicinity of the plate A. 



3. Experimental Arrangement. 



The diagram of connections and the disposition of apparatus is given in fig. 4 ; in 

 the main it is the same as that used by M. LANGEVIN, but certain necessary modifica- 

 tions were introduced to suit the special conditions. 



W and W are two iron weights which are supported by means of an electro- 

 magnet ; when the circuit thrcmgh the magnet is broken, the weights fall simultaneously 

 and break the platinum contacts at K and K' respectively. The breaking of the 

 contact at K', which is in the primary circuit of a Marconi induction coil, gives rise to 

 an induced E.M.F. in the secondary, and causes a momentary discharge to pass in the 

 Rontgen-ray bulb X ; the breaking of the contact at K reverses the potential of the 

 lower plate B of one of the chambers, as can be readily seen from the diagram, R 

 being a water resistance of the order of 1 megohm and B at the time of breaking 

 being at the same potential as the point /3. K' can be fixed at any point of a vertical 

 scale, thus enabling the potential of B to be reversed at any convenient interval 

 after the passage of the discharge. This vertical scale was graduated and the 



