6 
MR. LATIMER CLARK ON A STANDARD VOLTAIC BATTERY. 
diameter wound on a cylinder of ebonite, the ends being connected to the axes b b', 
which work in blocks of metal with mercury contacts : two batteries are also connected 
to the same blocks ; the larger one, C, of several cells, sends a continuous current through 
the coil, the strength of which can be varied by means of the rheostat or resistance-coil, d ; 
the smaller, c , is the standard element ; it is connected with the terminal blocks, b b', and 
it has a reflecting galvanometer, g, in circuit with it ; as these two batteries are connected 
up in the same direction, they both tend to send a current through the coil a a. If the 
difference of potential maintained by the battery between the blocks b V be greater than 
that of the standard cell, the battery will of course overpower the cell and send a 
reversed current through it ; if, on the other hand, the difference of potential be less, 
then both the battery and the cell will jointly send a current through a a. In practice, 
however, the resistance, d , is so adjusted that the difference of potentials at b and V is 
exactly the same as the difference of potential between the poles of the standard cell — 
in other words, is equal to its electromotive force, in which case no current passes 
through the galvanometer, g, and the cell remains inactive. 
In comparing a trial cell with the standard, one pole of the cell is connected with 
that end of the coil to which the similar pole of the standard is fixed ; the other pole 
is connected through a second galvanometer, h, to a sliding piece, i. By means of this 
sliding piece contact can be made at any point of the coil, a a , which is calibrated into 
10,000 equal divisions. The point along the wire is readily found at which the poten- 
tial is the same as that of the trial cell, and consequently no current passes through the 
galvanometer, h ; in this case the reading or number of divisions gives the value of the 
trial cell in ten-thousandth parts of the standard element. As it is necessary that the 
standard element should have a higher electromotive force than that which is compared 
with it, two or more cells may be employed as a standard. 
Having thus obtained a constant and easily reproducible measure of electric potential, 
it became necessary to ascertain its precise value in terms of the British-Association 
units and in absolute measure. There are two well-known methods ^ which this may 
be accomplished; the one is by the use of Weber’s electrodynamometer *, and the other 
by means of the sine or tangent galvanometerf, in which the force of the current, 
acting on a suspended needle through a known resistance, is compared with that of the 
earth’s horizontal intensity. It was determined to measure the element by both 
methods. 
The electrodynamometer employed was an instrument constructed for the British- 
Association Committee, and referred to in their Report for 1867, page 478. This 
instrument had not been previously used. 
In the electromagnetic system, the unit length of the unit current, acting on another 
similar current at the unit distance, exercises the unit of attractive or repulsive force. 
The value of the current (C) in absolute units may therefore thus be determined from 
its mechanical effect ; and the resistance (R) of the circuit being known, the value of the 
* Taylok’s Scientific Memoirs, yol. iii. f British-Association Report, 1863, pp. 116, 141. 
