Reversibility of Voltaic Cells. 



493 



cell against the accumulators it is easy to so adjust the current 

 that 1 millimetre of the bridge-wire shall correspond to a 

 change in E.M.F. of 1 millivolt. The bridge-wire is 1 metre 

 long, and thus corresponds to 1 volt. Also the coils (1) and 



Fiar. 2. 



(2) (see figure) are each of the same resistance as the bridge- 

 wire, and thus each of these corresponds to 1 volt. So that 

 it is very easy to get readings of any E.M.F. up to 3 volts. 



The scale-wire extends to B, and the wire AB had a resist- 

 ance equal to that of 43"4 millimetres of the scale-wire. Thus 

 if the reverse current was taken between A and D, its E.M.F. 

 was 2*0434 volts. If, however, as was sometimes the case, 

 the coil (1) was taken out of the circuit, B was equal to 

 1-0434 volts. 



The cell consisted of two glass vessels (short boiling-tubes) 

 which had side-tubes blown in them. These side-tubes were 

 connected by a short piece of india rubber tube, to which was 

 attached a screw-clip. It was found that, unless wads of 

 cotton-wool were placed in the connecting-tubes, it was neces- 

 sary to have solutions whose densities only differed in the 

 fourth place. With the wads there is practically no diffusion, 

 and the internal resistance is not unduly high. The solutions 

 were made up approximately to the formula R . 100H 2 O, 

 where R is the molecular formula of any salt. The densities 

 were then adjusted until they were approximately equal. 



Solution. 



Density at 

 13°-5 C. 



Solution. 



Density. 



Temp. 



o 

 25 



16 



25 



Copper sulphate ... 

 Cadmium „ 

 Zinc „ 



1081 

 1-082 

 1-082 



Copper chloride ... 

 Cadmium „ 

 Zinc ,, 



1-074 

 1-079 

 1-076 



