458 PEOFESSOE W. WATSON ON A DETEEMINATION OF THE VALUE OF 
voltameter. In the silver depositions fresh solution was always used, and the silver 
was always deposited in basins which had already a coating of silver. The value for 
the electro-chemical equivalent obtained was 0‘0011185. This number has to be 
reduced by 1 part in 10,000 if the deposit takes place on a platinum surface, as has 
been shown by Kahle. Thus for a deposit made on a platinum surface the value of 
the electro-chemical equivalent is 0‘0011184. Kahle also measured the E.M.F. of a 
cadmium cell by comparison with the Clark, and then used this cell to measure the 
electro-chemical equivalent of silver. In this way he obtained the va,lue 0‘0011183 
for a deposit on silver, or 0'0011182 for a deposit on platinum. Thus the mean value 
for a deposit on platinum is 0‘0011183. 
Taking into consideration that these measurements of Kahle’s appear to have 
been made with great care and with the resources of the Keichsanstalt, and further, 
that the conditions under which the silver was de230sited are well defined and are the 
same as those used by the author, it would seem that the best available value for the 
electro-chemical equivalent at the joresent date is 0’0011183. 
A consideration of what has been said alcove will show what grave objections there 
are to the silver voltameter as a means of measuring a current where an accuracy 
greater than 1 part in 1000 is required. For this reason Kahle has recommended 
that a standard cell (Clark or cadmium) together with a known resistance is a much 
more trustworthy means of measuring a current. Now in the experiments described 
in this paper the resistance coils employed were compared together immediately after 
the magnetic experiments, and then coil No. 1941 was sent to the Keichsanstalt to 
1)e compared with the German standards. Hence, knowing the values of the resis¬ 
tances used in the potential circuit, we can calculate the current emj)loyed in the 
magnetic experiments if we know the E.M.F. of the cells. 
In order to employ this method a second set of cadmium cells'^ and of Clark cells 
were prepared according to the German directions, from a fresh batch of chemicals 
obtained from Kahlbaum by Mr. F. E. Smith, one of the Demonstrators in the 
Physical Laboratory at the Ptoyal College of Science. This new batch of cadmium 
cells was compared with the old cells, and they were found to have an E.M.F. 
0‘00017 volt higher than the old. The Clark cells were compared with the cadmium 
cells by means of a potentiometer arrangement which contained one standard 
1000-ohm coil, three standard 100-ohm coils, one standard 1-ohm coil, and a box 
containinof 1 ohm s.ub-divided into tenths. Two accumulators sent a current through 
these coils placed in series and through a box of resistance coils and a carbon 
adjustable resistance. Two potential circuits were arranged which could by means of 
a switch be connected to the galvanometer in turn. One of these cii'cuits included 
* These new cadmium cells had as negative poles an amalgam containing 12-7 jier cent, of cadmium, 
while in the old cells the amalgam contained 14 per cent, of cadmium. The reason for the change is 
that the amalgam containing the smaller percentage of cadmium is more stable (see ‘ Drude Ann.,’ vol. 3, 
p. 3G6, 1900). 
