﻿Determination of the Ratio of the Electrical Units. 101 



possible, though probably more elaborate, to determine v 

 without water by the behaviour of included air. 



The investigation of the formula for the electromagnetic 

 measure of the capacity as derived from observations with 

 Wheatstone's bridge is given in Maxwell's ' Electricity/ 

 §§ 775, 776, but so succinctly that the full bearing of it may 

 easily be misunderstood. Thus Thomson * speaks of it as 

 " only an approximation/' and substitutes a fuller treatment. 

 After pointing out that in simple circuit the combination 

 of commutator (period T) and condenser (capacity C) is 

 equivalent to a resistance 1{, where R=T/C f, Maxwell pro- 

 ceeds to consider the bridge arrangement. " Let us suppose 

 that a zero deflexion of the galvanometer has been ob- 

 tained, first with the condenser and commutator, and then 

 with a coil of resistance R x in its place, then the quantity 

 T-r [2]0 will be measured by the resistance of the circuit of 

 which the coil R x forms part, and which is completed by the 

 remainder of the conducting system including the battery. 

 Hence the resistance E, which we have to calculate, is equal 

 to R 1? that of the resistance-coil, together with R 2 > the resist- 

 ance of the remainder of the system (including the batterv), 

 the extremities of the resistance-coil being taken as the 

 electrodes of the system." 



" Using the notation of Art. 317 [see figure], and supposing 

 the condenser and commutator substituted for the conductor 

 AC in Wheatstone's Bridge, and the galvanometer inserted in 



OA, and that the deflexion of the galvanometer is zero, then 

 we know that the resistance of a coil, which placed in AC 

 would give a zero deflexion, is 



•-!-*. 



(3) 



* Phil. Trans. 1883, p. 708. 



f Maxwell has 2 C in place of C, inasmuch as he supposes the charge 

 of the condenser to be reversed instead of merely annulled. 



