258 MR. E. M. WELLISCH ON THE MOBILITIES OF THE 



A single reading was then taken in the following manner (the null method being 

 employed) : 



(i) K' (fig. 4) is adjusted to the required height on the scale. 



(ii) A and A' are connected together by means of the key Y and are earthed ; the 

 quadrants also are at zero potential. In the key Z, b and m are connected, as also 

 are b' and p. The contact K is made. 



(iii) The magnet circuit is completed at F ; the weights W and W are placed in 

 position ; the movable arm D of the relay M makes contact with P, thus putting B at 

 zero potential. 



(iv) A and A' (which throughout remain connected) are insulated ; the quadrants, 

 however, remain earthed. 



(v) The contact K' is made ; the magnet circuit is broken at F ; the arm D makes 

 contact with Q, thus bringing B to the required potential ; the contact K' is broken 

 by W, thus giving rise to a flash of Rontgen rays which ionises the gas in each 

 chamber ; after the desired interval the contact K is broken by W, thus reversing the 

 potential on the electrode B. 



(vi) By the time the weights have fallen, the ions will have had sufficient time to 

 be all received at the electrodes ; the key connecting b and m (in Z) is then removed 

 and placed so as to connect b and e, thus bringing B again to zero potential. 



(vii) The quadrant pair is insulated ; the deflection due to the total charge received 

 by A and A' is then observed and noted. 



When the potential V is established on the electrode B, the upper electrode system 

 AA' will be raised to a potential r proportional to V. The value of v corresponding 

 to any definite potential V was determined experimentally in the following manner. 

 The upper electrode system was connected to the electrometer and the steady deflection 

 d was noted when B was raised to the potential V. The quadrants were then earthed 

 and the upper electrode system was charged to a known small potential v' by means 

 of a potentiometer ; the system was then disconnected from the potentiometer and 

 connected to the insulated uncharged quadrant pair, causing a steady deflection d'. 

 The potential v' was then varied until d' became equal to d ; this value of v' 

 corresponds to the required potential v. As a result of a series of observations with 

 different values of V, it was deduced that when the electrode B was raised from zero 

 potential to a potential of V volts, the upper electrode system attained a potential of 

 0-0425 V volt.* 



The corresponding electric field, which was sensibly uniform in the region to which 

 the ionisation was confined, was thus 0'9575 V volt per centimetre. 



* The potential v should also be given by fl/diS, where d denotes the steady deflection as above, d\ 

 denotes the steady deflection resulting from reconnecting the upper electrode system to the quadrants after 

 they have been earthed and insulated, and 8 denotes the sensitiveness of the electrometer over the range 

 of deflections under consideration. A series of observations led to the same result as above. 



