[707] 
XXIII. On the Determination of the Number of Electrostatic Units in the 
Electromagnetic Unit of Electricity. 
By J. J. Thomson, M.A., Fellow and Assistant Lecturer, Trinity College, Cambridge. 
Communicated by Lord Rayleigh, F.R.S. 
Received June 19,—Read June 21, 1883. 
The values which various physicists have found for “ v,” the number of electro¬ 
static units in the electromagnetic unit of electricity, differ so widely from each 
other that it seems important that additional experiments should be made in order to 
help to determine the value of this important constant. Six determinations of “ v ” 
have been published. The first determination was made by Weber, who measured the 
capacity of a condenser, both electrostatically and electromagnetically. Hockin and 
Professors Ayrton and Perry have also determined “v” in this way. Maxwell 
determined it by balancing the electrostatic attraction between two discs maintained 
at different potentials against the repulsion between electric currents circulating at the 
back of the discs, the currents being derived from the battery which maintained the 
discs at different potentials. Sir William Thomson and Mr. Shida have determined 
it by measuring an electromotive force both electrostatically and electromagnetically. 
The following method was employed in this investigation: it is a very slight 
modification of the method described in § 776 of Maxwell’s ‘Electricity and 
Fig. 1. 
Magnetism.’ In a Wheatstone’s bridge, A B C D, with the galvanometer at G, and 
the battery between A and B, the circuit B D is not closed, but the points B and D 
are connected with two poles, R and S, of a commutator, between which a travelling 
piece, P, moves backwards and forwards; P is connected with one plate of a con¬ 
denser, the other plate of which is connected with D. Thus when P is in contact 
