Manchester Memoirs, Vol. Ix. (191 6), No. 1 



1 i 



e. The back E.M.F. method. 



This method was originally suggested by Le Blanc, 

 and largely modified by Dr. Pring and the author of this 

 paper. 



The apparatus in its latest form is shown in Text-fig. 5. 



The battery B 2 sends a current through the potentio- 

 meter P. v and by means of movable keys K z and K± any 

 required voltage may be tapped off and applied to the 

 electrodes A and C through the milliammeter M. The 

 voltage applied may be observed by means of the volt- 

 meter V. 



By means of a rapidly rotating commutator X, the 

 cathode C is first connected to this source of current, and 

 then the circuit is broken and the cathode at once con- 

 nected with the potentiometer circuit P v the standard 

 hydrogen electrode H being now the anode. The key iT 2 

 is moved until no deflection is observed in the galvano- 

 meter G when the key K 5 is depressed, and this reading 



