ELECTRICAL APPARATUS IN USE IN PHYSIOLOGICAL. WORK 5 



joined electrical equilibrium tends to re-establish itself in the circuit 

 thus closed. It is common to speak of a current as flowing from the 

 anode to the kathode outside the cell and from the zinc to the copper 

 inside. 1 The amount of this current depends upon the difference of 

 potential produced within the cell. This is diminished by any 

 increase of resistance to the flow of electricity, whether occurring 

 within the cell or in the outside circuit. Electromotive force (E.M.F.) 

 is measured in volts; thus the E.M.F. of a Daniell cell is 1-079 volts. 

 It may be increased by coupling two or more cells together in series, 

 the zinc of one connected with the copper of the next, and so on. 

 When cells are coupled together, they form what is known as a voltaic 

 or galvanic battery. 



FIG. 2. Diagram of a voltaic couple. Z, Zinc ; C, Copper. 



If electricity be generated simply by immersing plates of zinc and 

 copper into acid the chemical action which ensues causes bubbles of 

 hydrogen gas to form on the copper, and this not only introduces a 

 resistance to the flow of current through the cell, but the hydrogen 

 tends to set up a current (polarisation current) in the opposite direction 

 in the cell and circuit ; from both these causes the original E.M.F. 

 of the cell becomes rapidly weakened. 



To obviate this effect Daniell placed the copper plate in a saturated 

 solution of copper sulphate and introduced a porous pot to separate 

 this from the dilute sulphuric acid in which the zinc is immersed (Fig. 

 3). The zinc then dissolves in the acid, displacing hydrogen; 2 the 

 hydrogen in its turn displaces copper from the copper sulphate, and 

 the displaced copper is deposited on the copper plate, so that no 

 bubbles of hydrogen are formed upon the metal, and if the copper 

 sulphate solution is kept saturated, the E.M.F. of the cell remains 

 constant. Commercial zinc, which is never pure, must always be 



1 Within the. cell the electrical potential is highest at the zinc, which is there- 

 fore here the anode, and lowest at the copper, which is here the kathode. 



2 A solution of zinc s^^lphate being formed. It is convenient to use a saturated 

 solution of zinc sulphate in the porous pot in substitution for the dilute sulphuric 

 acid. 



