of Silver and of Copper. 413 



convenient to solder separate terminal pieces to one end of 

 each wire, so that the number in each group may be varied 

 by putting the terminals in one or other of two mercury 

 troughs. Between the bars b± and b 2 a zero galvanometer g 

 is connected, and is used for the purpose of indicating when 

 Z> 1 and b 2 are at the same potential. When this is the case, 

 the current from b to bi is to the current between b and b 2 in 

 the ratio of the resistance between b and b 2 to the resistance 

 between b and 6j. Two sets of resistances, or two rheostats, R 

 and R x , are introduced into the branches for the purpose of ad- 

 justing the currents to the required strength. The electrolytic 

 cells E and E t are placed in one or other of the branch circuits 

 according as the instrument to be graduated is designed for weak 

 or for strong currents, and the instrument is included in the 

 other branch. The circuit is closed through a suitable battery, 

 which must be of such a kind as will maintain a nearly uniform 

 electromotive force. The current through the instrument Gi is 

 kept constant by means of the rheostat E^, and the difference of 

 potential between b x and b 2 is kept at zero by means of the 

 rheostat R. When the instrument Gi is to be standardized by 

 means of a standard galvanometer or a standard current-balance, 

 the arrangement and mode of operation is precisely similar, the 

 standard instrument taking the place of the electrolytic cells. 



in 



Note on the Effect of Excessive Current-Density at the Anode 

 the Electrolysis of Copper Sulphate. — On page 396 above, 

 the effect of excessive current-density at the anode of a copper- 

 copper-sulphate electrolytic cell is referred to, and it is there 

 stated that, unless the anodes present a surface greater than 40 

 square centimetres per ampere, the current is apt to diminish 

 greatly in strength after the first few minutes. 



As the result of some special experiments on this subject I 

 find that the requisite size of anode depends greatly on the 

 degree of saturation of the solution. When the solution is 

 nearly saturated, say above 1*18 in density, the current is apt 

 to be almost entirely stopped on account of the anode plates 

 becoming completely covered by a finely crystalline deposit 

 of copper sulphate which dissolves in the liquid very slowly. 

 A current of one-tenth of an ampere derived from a battery 

 of 24 tray Daniells was passed through an electrolytic cell 

 charged with a solution of sulphate of copper of density 1*18, 

 and having two anode-plates made of round copper wires 

 presenting a total surface of 3*8 square centimetres. The 

 current remained nearly constant for 12 minutes, when it 

 gradually diminished, became practically zero in a few seconds, 

 and remained so for 20 minutes. At the end of this time the 



