252 MEASUREMENT OF CURRENTS. 



The introduction of a galvanometer of very great resistance 

 between two points A and B is equivalent to the use of an elec- 

 trometer, the electrodes of which are connected to the same points. 

 The electrometer, once graduated for a standard electromotive force, 

 gives the difference of potential E, from which is deduced the in- 

 tensity / of the current. 



As a particular case, if we use the quadrant electrometer as 

 in (816), connecting the needle with one pair of quadrants, and 

 those separately to the points A and B, then if k is a constant for 

 the instrument, the deflection 8 may be represented by the formula 



If 8 is the deflection produced by the standard electromotive 

 force E , we have again 



k 



~ -^o 



and therefore 



869. The difference of potential measured between two points 

 A and B by a galvanometer of great resistance, or by an elec- 

 trometer, may be due merely to the passage of a current in the 

 resistance, or it may comprise an electromotive force of any nature 

 whatever. In any case, knowing the value I of the current and the 

 difference of potential E of the points A and B, the product El 

 represents the electrical work between these two points. 



We may determine these factors separately by a galvanometer 

 of great resistance or by an electrometer. The apparatus is suc- 

 cessively connected with two points A and B, comprising the total 

 electromotive force E, and with two other points A' and B', simply 

 separated by a known resistance R. 



The work El may be directly estimated by the quadrant electro- 

 meter.! If V 15 V 2 , Vj, and V 2 are the respective potentials of the 



* JOUBERT. Comptes rendusy Vol. XCI., p. 161. 1880. Annahs de VEcole 

 Normale [2], Vol. x., p. 131. 1881. 



t POTIER. Journal de Physique, Vol. IX., p. 445. 1881. 



