ELECTRO-CHEMICAL EQUIVALENT OF SILVER. 
417 
The mean square root of the numbers thus obtained gives us a result to which the 
rate of silver deposit should be proportional. 
§ 9. The use of a balance for the measurement of electromagnetic attraction involves 
some special arrangements. The suspended coil must in every case be brought 
to rest in its proper position, corresponding to the zero of the pointer of the balance. 
It was found desirable to give the balance a shorter period of vibration than usual, 
and to obtain control over the arc of vibration an auxiliary coil was introduced, 
through which, with the aid of a key, the current from a Leclanche cell could be 
made to pass. By this means a force tending to raise or to lower the suspended parts 
could be brought into play at the will of the operator, who, after a little practice, is 
able to stop the vibrations with very little delay.* The weighings were recorded to 
milligrams only; but the accuracy really obtained was greater than might appear, 
since by anticipating somewhat the change in progress it was possible to note the time 
at which the balance demanded an integral number of milligrams. 
The current was led into the suspended coil by means of fine flexible copper 
wires. To diminish the force conveyed by these to the suspended parts, they were 
bent so as to place themselves naturally in the required positions before the final 
solderings were made. It is important, however, to observe that no assumption is 
made as to the equality of these forces before and during the passage of the current. 
Under its influence the fine wires are no doubt sensibly warmed, but this effect 
and any consequent alterations in the mechanical properties are the same in both sets 
of readings, the only change relating to the direction of the current in the fixed 
coils. 
This point is the more important since the balance is not used in these experiments 
in quite the normal manner. In ordinary weighings there is no force in operation 
upon the pans but gravity, and this vertical force is transferred to the beam. In the 
present application the “ pan ” is not quite free and is subjected to forces which may 
have a small horizontal component. In virtue of the freedom of rotation about the 
knife-edge suspending the j3an, these forces are transferred without change to the 
beam. The horizontal component would, however, produce little effect in any case, 
since in the horizontal position of the beam its direction would pass very nearly 
through the knife-edge supporting the beam. The weights in the other scale-pan 
give rise to a strictly vertical force. We shall thus be doubly secured against error if 
we provide that the force to be measured (due to the reversal of the current in the 
fixed coils) is strictly vertical, and that the horizontal force, if sensible, remains 
unaltered in passing from one direction of the current to the other. These objects 
are attained when the coils are carefully levelled, and when the readings are always 
taken for a definite position of the suspended coil conveying a constant current. 
§10. The suspended coil is wound upon an ebonite ring (§ 13), and is supported by 
* See “ Suggestions for Facilitating the Use of a Delicate Balance.” B. A. Report, 1883, 
MBCCCLXXXIV. 3 H 
