434 
LORD RAYLEIGH AND MRS. H. SIDGWICK ON THE 
to give the details of one experiment. Thus on March 10, 1884, the current, roughly 
regulated to the desired value with the aid of the tangent galvanometer, was allowed 
to pass through the coils of the current-weighing apparatus for about half an hour. 
The electromotive force of the storage cells (when in good order) remains almost 
perfectly constant during an experiment, hut the gradual warming of the copper con¬ 
ductors causes a slight falling off of current. On the present occasion the preparatory 
current was a little stronger than that ultimately used, so as to produce a slight over¬ 
heating. During this time the three platinum voltameters, previously cleaned and 
weighed, were charged with solution of silver nitrate ; and the pure silver anodes, 
wrapped in filter paper, were adjusted to their places at the top of the licprid. As 
will be seen from Table II., two of the bowls were charged with solution of normal 
strength (15 per cent.), and the other with solution of double this strength. When 
all was ready, the current, previously running along a shunt, was caused to pass 
through the voltameters at 4 h 17 ra by the chronometer. The weights required to 
bring the pointer of the current-weighing balance to zero, with the corresponding 
times, are given in Table III. In the second column the first number means that at 
Table III. 
Time. 
Weight. 
Time. 
Weight. 
h. 
m. 
s. 
h. 
m. 
S. 
4 
19 
30 
7-694 
4 
25 
0 
6-795 
4 
32 
15 
7-698 
4 
40 
20 
6-791 
4 
42 
50 
7-699 
4 
50 
30 
6-790 
4 
53 
10 
7-699 
4 
56 
30 
6-789 
5 
1 
15 
6789 
the moment in question the weight required to balance the suspended coil, as acted 
upon electromagnetically, was 7'G94 grms., or rather 577'694 grms., but the 570 grins, 
being never moved need not be recorded. In this position of the reversing key the 
electromagnetic force increased the apparent weight of the suspended coil. The other 
set of readings, in which the magnetic force tended to lift the coil, are given in the 
fourth column. At 5 h 2 m the circuit was interrupted. 
From the numbers above given two curves are constructed (Plate 17, fig. 4),representing 
what would have been observed in either position of the key during the whole course 
of the experiment. To effect the integration of the current, the whole time, 45 ra , is 
divided into nine periods of 5 m each, and the magnitude of the current at the middle of 
each period is taken to represent its value throughout the period. A more elaborate 
evaluation could easily have been applied, but was superfluous. The difference of 
ordinates at the middles of the periods gives the difference of weights in the second 
column of Table IV., and the mean of the square roots of these differences, viz. '95171, 
is the square root of the difference of weights corresponding to the mean current. 
