METHODS OF ELECTRICAL STIMULATION 53 



rent. On cutting off the electrolyzing current, 

 the polarization current may be measured. 



Note the position of the meniscus of the capil- 

 lary electrometer. Turn the pole-changer so that 

 the cell is cut off and the electrodes are brought 

 into the electrometer circuit. 



The meniscus will indicate a current opposite 

 in direction to the current from the cell. 



Dry Cell. A "dry" cell is very convenient 

 for large classes. It usually consists of a zinc 

 cup, lined with plaster of Paris, saturated with 

 ammonium chloride, in the centre of which is a 

 carbon plate surrounded with black oxide of 

 manganese. When the cell is in action, the zinc 

 forms a double chloride of zinc and ammonium 

 while ammonia gas and hydrogen are liberated 

 at the carbon pole. These cells should never be 

 used continuously for many minutes, for they are 

 rapidly polarized by the accumulation of hydro- 

 gen on the carbon plate. The unused cell re- 

 gains its difference of potential by the union of 

 the hydrogen with the oxygen slowly given off 

 by the manganese dioxide, which therefore acts 

 as a depolarizer. 



INDUCTION CURRENTS 



A most useful method of electrical stimulation 

 of living tissues is by the induced current, and 



