METHODS OF ELECTRICAL STIMULATION 49 



and copper. The amalgamated zinc is placed in 

 a porous cup filled with dilute sulphuric acid. 

 The copper is placed in a solution of copper sul- 

 phate kept saturated by crystals of the salt. 

 When the circuit is closed, the zinc "dissolves" 

 in the sulphuric acid, carrying with it the elec- 

 tricity with which the zinc ions are charged. 

 The electricity is carried through the solution 

 by the migration first of hydrogen and then of 



Fig. 11 A. 



Fig. 11 B. 



A, earlier form of pole-changer. The rubber handle prevents the cross- 

 ing of the current from one side cup to the other. B, diagram of pole- 

 changer arranged (1) to change the direction of the current, (2) as a double 

 key, without cross-wires, (3) as a simple key. 



copper ions. It leaves the solution at the cath- 

 ode where the copper ions are converted into 

 metallic copper and deposited on the cathode. 

 The quantity of zinc dissolved and copper de- 

 posited is proportional to the quantity of the 

 current. One ampere deposits per minute 19.75 

 milligrams copper, and dissolves 20.32 milligrams 



zinc. 



4 



