ELECTROMETRIC MEASUREMENTS. 415 



resistance is N = + ', and the difference of potential of the 



terminals of the former is a fraction equal to , = of that of 



n + n N 



the extreme terminals. It is advantageous in this case to vary the 

 two numbers n and ', so that the sum n + ri = N remains constant, 

 and equal to 10,000 ohms, for instance. 



1009. ELECTROMETRIC MEASUREMENTS. Electromotive forces 

 may be determined in absolute or relative value by the various 

 methods pointed out in Chapter I. (976 et seq.} for determining 

 differences of potential, as well as in numbers 868 and 869. 

 With batteries and permanent currents, these methods are generally 

 simpler than in experiments in statical electricity, for the apparatus 

 themselves are sources of electricity, and losses of electricity 

 due to the connections are soon repaired, so that in general no 

 account need be taken of the capacity of the bodies concerned. 



It must be observed that the electrostatic methods give results 

 which are also measured in electostatic units, and that if we 

 wish to pass from one system to another (610) we must know 

 the ratio of the units (610). Sir W. Thomson* found, for instance, 

 that the electromotive force of a DanielPs element is equal to 

 0*00374 electrostatic units (C.G.S.) If we assume for the ratio 

 of the units the value a = $ x io 10 , this result corresponds to 

 0*00374 x 3. io 10 = 1*12. io 8 in electromagnetic units, or 1*12 volts. 



1010. OPEN ELEMENTS. METHOD OF OPPOSITION. In the 

 case of liquid elements, we should consider the electromotive force 

 from two points of view, according as the circuit is open or 

 closed, or again according as it has already been closed for 

 sometime. In all cases the electromotive force is the sum of 

 the differences of potential at the surfaces of contact of the 

 successive elements, which form each of the couples. But if the 

 battery has been traversed by a current, the chemical work has 

 modified by polarization the differences of potential, and the 

 electromotive force really put in play may have a totally different 

 value ; it is this latter which has more particularly a practical 

 interest. It is then important to specify the condition in which 

 the experiments have been made. 



The current is zero in a circuit in which the algebraical sum 

 of the electromotive force is zero that is to say, when the electro- 

 motive forces divide into two opposed groups forming the same 

 sum. In order to apply this method of direct opposition, we must 



* Sir W. THOMSON. Reprint of Papers on Electricity and Magnetism, p. 245. 



