184 ELECTRICAL CONSIDERATIONS. 



diagram of the simple cell (Fig. 20). The immersed portion of the zinc 

 is the positive plate, whilst the projecting part is the negative pole 

 (Kathode). 



The current flows from the zinc to the C plate in the fluid, and 

 circulates from the positive pole (Anode) to the zinc through the 

 external connection. The hydrogen which is liberated appears on the 

 negative C element, where it forms a film which is not only non- 

 conducting but is a strongly elqctro-positive as well. Tins has an 

 important bearing upon the working of the cell, as there is an increased 

 resistance offered to the passage of the current by the film of hydrogen 

 bubbles on the negative plate, which, at the same time, exerts an 

 electromotive force in a direction contrary to that of the cell, and 

 both taken together ultimately render the cell inoperative. This is 

 termed polarisation. A corresponding action takes place between metal 

 terminals (Electrodes) and the animal tissues with which they may be 

 in contact when a current passes through them. 



Amalgamation of the zinc plate. Pure zinc is not attacked by 

 the acid. Owing, however, to the presence of iron, arsenic, &c., local 

 circuits on the surface of the zinc come into action and the metal is 

 eaten into holes. To remedy this the zinc is amalgamated with 

 mercury. The plate is first cleaned with 1 in 10 sulphuric acid in 

 water until bubbles of gas are given off, and is then rubbed over with 

 mercury, using a stiff' nail brush for the purpose, so as to produce an 

 even covering, and to remove superfluous mercury. The impurities 

 are thus detached from the zinc and a more nearly pure zinc surface 

 is offered to the acid, and local action is largely diminished. Singing, 

 i.e., the formation of bubbles should be at a minimum when the circuit 

 is open. 



Galvanic cells differ in character in regard to roltaye, constancy of 

 action, and internal resistance. 



The voltage or E.M.F. of a cell depends upon the nature of its 

 constituent elements. 



The internal resistance depends upon (a) the area of the plates, 

 (b) the distance at which they are placed from each other, and (c) the 

 resistance of the sensitising fluid. 



The constancy of action depends upon the completeness with which 

 polarisation is abolished. 



The Daniell cell* This is the standard cell used for physiological 

 work. The zinc, with its acid, is separated by a porous septum from 

 the copper, which is placed in a saturated solution of copper sulphate 

 (the depolarising agent). The hydrogen liberated unites with the 



