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X, A Determination of“v,” the Ratio of the Electromagnetic Unit of Electricity 
to the Electrostatic Unit. 
By J. J. Thomson, M.A., F.R.S., Cavendish Professor of Experimental Physics, 
Cambridge, and G. F. C. Searle, B.A., Peterhouse; Demonstrator at the 
Cavendish Laboratory, Cambridge. 
Received March 12,—Read March 27, 1890. 
The experiments made by one of us in 1883 having given a value of “ -y ” considerably 
smaller than the one found by several recent researches, it was thought desirable to 
repeat those experiments. The method used in 1883 was to find the electrostatic 
and electromagnetic measures of the capacity of a condenser ; the electrostatic measure 
being calculated from the dimensions of the condenser, the electromagnetic measure 
determined by finding the resistance which would produce the same effect as that 
produced hy the repeated charging of the condenser placed in one arm of a Wheat¬ 
stone’s Bridge. In the experiments of 1883 the condenser used in determining the 
electromagnetic measure of the capacity was not the same as the one for which the 
electrostatic measure had been calculated, but an auxiliary one, without a guard ring, 
the equality of the capacity of this condenser and that of the guard ring condenser 
being tested by the method given in Maxwell’s ‘Electricity and Magnetism,’ vol. 1 , 
p. 324. 
In repeating the experiment we adopted at first the method used before, using, 
however, a key of different design for testing the equality of the capacity of the two 
condensers by Maxwell’s method. We got very consistent results, practically 
identical with the previous ones. We may mention here, since it has been suggested 
that the capacity of the leads might account for the small values of “ v ” obtained, that 
this capacity is allowed for by the way the comparison between the capacities of the 
auxiliary and guard ring condensers is made, for the same leads are used both in this 
comparison and in the determination of the electromagnetic measure of the capacity 
of the auxiliary condenser; the capacity of the auxiliary condenser, plus that of its 
leads, is made equal to the capacity of the guard ring condenser, and it is the capacity 
of the auxiliary condenser, plus its leads, which is determined in electromagnetic 
measure. As the introduction of the auxiliary condenser introduced increased possi¬ 
bilities of error, we endeavoured to determine directly the electromagnetic measure of 
21.10.90 
