﻿1036 Prof. Townsend and Mr. Bailey on the 



would be too small to have an appreciable effect in most of 

 the experiments as the electric forces were so large, but it 

 was considered advisable to reduce as far as possible any 

 error that might affect the experiments with the smaller 

 forces. 



In the original apparatus the electrodes were of unsilvered 

 brass, and when the experiments with hydrogen were repeated 

 with* the silvered electrodes almost exactly the same results 

 were obtained. 



3. The position of the slit in both instruments was 

 adjusted to bring the centre of the stream slightly to one 

 side of the centre of the electrode E 2 . This arrangement 

 makes the instruments very adaptable for the measurement 

 of the velocities in the direction of the electric force. 



For this purpose the stream is deflected by a transverse 

 magnetic force H which is adjusted to bring the centre of 

 the stream to coincide with one of the gaps between E 2 and 

 the electrodes E x and E 3 . The electrode E 2 was a flat strip 

 4*5 millimetres wide and each of the gaps *5 millimetre 

 wide, so that the distance between the centres of the two 

 gaps was 5 millimetres. Let this distance be 2a, b the 

 distance of the centre of the stream from the centre of 

 the electrode E 2 when H = 0, Hj the magnetic force required 

 to deflect the centre of the stream through the distance a + b, 

 so that the current received by E x is equal to that received 

 by E 2 and E 3 , H 3 the magnetic force in the opposite direction 

 which deflects the centre through the distance (a — b), the 

 current received by E 3 being then equal to that received by 

 E 2 and E x . The velocity W in the direction of the electric 

 force Z is given by the equations 



HjW_q-+& 

 Z e ' (1} 



H 3 W a-b 



-?r = ~7 ' v> 



where c is the distance from the slit to the electrodes E. 

 Thus b is determined by the relation H 1 /H 3 = (a + 6)/(a — b). 

 The magnetic field which was uniform in the space 

 between the slit and the electrodes E was produced by a 

 current in two large coils fixed in position on either side of 

 the apparatus. With the larger gas pressures the velocities 

 W are comparatively small, and it was convenient to deflect 

 the stream through the shorter distance a — b as the coils 

 became overheated when currents of the order of 15 amperes 



