during the Absorption of Electrons by Platinum. 177 



There will be a point M in S such that at this point the 

 current along S which arises from the thermions is zero. The 

 thermionic current will flow from M towards R l5 and towards 

 L, and from those points along the various possible connexions 

 to the point, wherever it may be, (in the present case at E) 

 where the thermionic current is led out of the circuit. It 

 will be seen that if the resistances R 4 and R 5 are introduced 

 as shown, provided the resistance from M through R t to K 

 is to the resistance from M to L as R 4 is to R 5 , K and L will 

 still be at the same potential even when the thermionic 

 current is flowing. 



The method thus involves a double adjustment which was 

 carried on as follows : — First of all the switch A was put to 

 the left, so that the thermionic current was off. The resistance 

 R x was then adjusted until no current with the battery C 1 C 2 

 on flowed through the galvanometer G, involving the usual 

 condition of proportionality between the arms S, R l5 R 2 , and 

 R 3 . The battery CjC 2 was then put out of commission, and 

 one of the resistances R 4 , R 5 was adjusted until no deflexion 

 of the galvanometer occurred when the thermionic current 

 was turned on. It might be thought that if the battery circuit 

 DC 2 was broken when the second adjustment was made, the 

 adjustment would not hold when the switch D was subse- 

 quently closed, owing to the resistance in this arm being- 

 different under the two sets of circumstances. To avoid this 

 objection two separate batteries Ci, C 2 were used, and it was 

 arranged by means of a commutating switch that they could 

 either be put in series or opposed to one another. In the 

 one case the potential in this circuit would thus be twice that 

 of a single battery while in the other case it would be zero, 

 but in either case the resistance of the arm DCxC^ would be 

 the same. In carrying out the first adjustment the batteries 

 Cj C 2 were in series and produced the bridge current, while in 

 carrying out the second adjustment they were opposed to one 

 another, so that there was no current actuating the bridge 

 circuit. As a matter of fact, we were not able to satisfy our- 

 selves that it made any difference whether the switch D was 

 open or closed when the second adjustment was made. 



In order to be quite certain that this method of com- 

 pensating for the effect of the thermionic current flowing 

 into the arm S was free from objection, we made dummy 

 experiments with a resistance in the arm S provided with a 

 sliding contact, so that small differences of potential from a 

 battery could be introduced between the contact and the 

 point E. The operations were then repeated with this 

 arrangement. First the resistance Ri was adjusted with the 



Phil Mag. S. 6. Vol. 20. No. 115. July 1910. N 



