CHAPTER I. 



ELECTRICAL APPARATUS FOR PHYSIOLOGICAL EXPERIMENTS. 



IN experimental physiology the stimulus most frequently used is an 

 electrical one, for it is convenient, easily graduated, and less injurious 

 to tissues than efficient thermal, chemical, or mechanical stimuli 

 would be. 



The Daniell Cell, which has an electromotive force (E.M.F.) of 1*1 

 volts, is the best source of electricity, for it yields an almost constant 

 strength of current. It consists (Fig. 1) of (i) a plate of copper dipping 



into a solution of copper sulphate 

 which is kept saturated by crystals 

 of the salt, and (ii) a rod of amal- 

 gamated zinc placed in a porous pot 

 filled with a 10 per cent, solution of 

 sulphuric acid ; the porous pot is 

 surrounded by the solution of copper 

 sulphate. The whole is generally 

 placed for convenience in a glazed 

 earthenware pot with a handle. 



When the copper and zinc 

 elements are connected by a wire 

 the zinc dissolves in the sulphuric 

 acid, forming ZnS0 4 -rH 2 . The H 

 ions thus liberated become charged 

 with the electricity originally stored 

 in the zinc; they migrate through the porous cell into the copper 

 sulphate and split it up into H 2 S0 4 + Cu, and their charge of electricity 

 is transferred to the Cu ions. These in turn deliver up their charge 

 of electricity to the copper plate and, as they discharge, become 

 deposited on the plate as metallic copper. 



Thus inside the cell electricity passes from the zinc, or positive 



FIG. 1. Diagram of a Daniell cell seen in 

 section. 



