of Silver and of Copper, 409 



the cathode; but both sets agree in indicating rather less than 

 a tenth per cent, as the correction which has to be added to 

 the results obtained with the denser of these currents, in order 

 to arrive at the true value of the electro-chemical equivalent 

 of copper. 



The results of Table IV. give for the amount of copper 

 deposited by a coulomb of electricity, when the cathodes ex- 

 pose a surface of fifty square centimetres to the ampere, 

 •0003287 gramme, if we assume '001118 as the correspond- 

 ing number for silver. They would thus indicate a value not 

 differing much from '0003290 as the true value of this con- 

 stant for copper. In the use of copper for the measurement 

 of currents by electrolysis the absolute value of this latter 

 number is not of much importance; what is wanted is the 

 proper number to use for a certain current-density, and at 

 ordinary temperature this number will not differ much from 

 the number obtained by using '0003287 for cathodes of fifty 

 square centimetres per ampere, and correcting for other sizes 

 by the dotted line in curve 13. 



A few experiments were made with very weak current- 

 densities, the circuit being kept closed for about a week in 

 each case. The results show that when the current-density is 

 very small the rate of loss by direct action of the liquid is 

 much the same as if no current were flowing. The deposit 

 was patchy and did not usually cover all the surface of the 

 plate, a result which was perhaps due to inequalities in the 

 plate itself. 



The difference between the gain and loss of weight by the 

 plates in the electrolytic cell is usually very much greater 

 than can be accounted for by the loss of similar plates placed 

 in the same liquid when no current is flowing, a result which 

 appears to be largely contributed to by the anode-plates losing 

 very much more when the current is flowing than when it is 

 not. If the difference between the gain and loss be divided 

 by the sum of the areas of the anodes and cathodes, and the 

 quotient, multiplied by the area of the cathode, be added to 

 the gain, a result is obtained which is always too high to give 

 the true electro-chemical equivalent, but which is very nearly 

 constant for different cells, even when they begin to give 

 uncertain results from the gain of weight taken by itself. 



Arrangement of the Circuit. — For experiments with weak 

 currents, such as those the results of which are given in the 

 Tables II. to V., the circuit generally included a battery of 

 twenty- four tray cells, the electrolytic cells, one of Sir William 

 Thomson's improved rheostats, and a galvanometer. The 

 galvanometer was in some of the earlier experiments one of 

 Thomson's lever voltmeters, but in the later experiments one 



