416 
LORD RAYLEIGH AND MRS. H. S1DGWICK ON' THE 
We may observe that an error in the number of windings, or, which comes to the 
same thing, a defect of insulation, produces a more serious effect in the case of the 
suspended than in the case of the fixed coils. The error in the ratio of the galvano¬ 
meter constants enters proportionately, but the error in the weighings is halved. 
Full details of the coils are given later. It will be sufficient here to say that the 
radius of the large coils is about 25 centims., and that of the suspended coil about 
10 centims. The total number of windings on the fixed coils is 450, and on the 
suspended coil 242. The current usually employed was about ^ ampere, and the 
double attraction was about the weight of one gram.* 
§ 8. The double attraction is spoken of, inasmuch as the readings were always taken 
by reversal of the current in the fixed coils, for which purpose (Plate 17, fig. 1, E) a 
suitable key was provided. The difference of the weights required to balance the sus¬ 
pended parts in the two cases gives twice the force of attraction between the suspended 
coil and the fixed coils, independently of the action upon the former of any other part of 
the circuit, and of terrestrial or other permanent magnetism. The current was supplied 
from about 10 either Grove or secondary cells A, and traversed in succession a rough 
tangent galvanometer D (convenient for a preliminary test of the strength and direction 
of the current), two or more silver voltameters in series C, the suspended coil G, and 
then (of course, in opposite directions) the two fixed coils F. The weights necessary for 
balance (in the same position of the key) alter somewhat, both on account of variation 
in the electric current and also from the formation of air currents, due to a slight 
progressive warming of the suspended coil. By recording the times of each weighing 
we can plot two curves (§ 24), from which we can find what would have been at any 
moment the weighing in either position of the key. The difference of ordinates gives 
us what we should have observed, were it possible to make both measurements 
simultaneously. The whole duration of an experiment was from three-quarters of an 
hour to two hours, measured by a chronometer, and as a weighing could be taken 
about every five minutes there was ample material for the construction of the curves. 
What we require for comparison with the deposited silver is the mean current, whereas 
what we should obtain directly from the curves represents the square of the current. 
The whole interval is divided into periods (usually of fifteen minutes), and the difference 
of ordinates corresponding to the middle of the periods is taken from the curves. 
* The actual apparatus was not adapted to the measurement of currents much exceeding 4 ampere. 
The flexible copper connexions of the suspended coil would take an ampere, but the suspended coil itself 
is unduly heated by the passage of an ampere for more than a few minutes. Had it been desirable to 
use stronger currents, it would, of course, have been possible to do so by increasing the gauge of the wire. 
The grooves in which the wire is wound being given, it is evident that a proportional increase of the 
current and of the section of the wire leave both the heating and the electromagnetic effects unaltered. 
In this way the apparatus might easily be modified, so as to take currents of 3 or 4 amperes, the only 
other changes that would be required being a multiplication of the flexible leading wires, several of 
which might be arranged in parallel. But for the determination of the electro-chemical equivalent of 
silver, the currents actually used were quite strong enough. 
