ELECTRICAL APPARATUS IN USE IN PHYSIOLOGICAL WORK 5 



negative electricity ; these ends are called the positive pole, or anode, 

 and the negative pole, or kathode, respectively. The anode is said to 

 be in a condition of higher potential and the kathode in one of lower 

 potential, and when they are 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 

 battery 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 T079 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. 



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 

 being electropositive 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 ; the hydrogen 

 in its turn displaces copper from the copper sulphate, and the dis- 

 placed 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 " amalgamated " 



1 Within the battery the electrical potential is highest at the zinc, which is 

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



