REGULATION OF A GALVANOMETER. 259 



As this intensity is sensibly proportional to the deflection, if N is 

 the number of divisions corresponding to unit current, 



If Rj is the resistance which the circuit should have, so that the 

 standard electromotive force produces a deflection of one division, 

 we have 



- r 



The coefficient Rj measures the sensitiveness ; it is often called 

 the figure of merit of the galvanometer. 

 We have further 



E 



With very sensitive galvanometers such as are used at the 

 present time, the resistance R is generally very great in respect 



or 



of , and we may simply write 



If the electromotive force E is expressed in volts and the 

 resistance R in ohms, the intensity / is in amperes. 



879. When the resistance R is not very great, the internal 

 resistance of the couple cannot be neglected; but this source of 

 error may be eliminated by one of the following methods, due to 

 the late Mr. Charles Hockin.* 



The arrangement of the apparatus being represented by 

 Fig. 1 68, we note the deflection of the galvanometer G provided 

 with a shunt s. If a, b^ and c are the resistances of the three 



* HOCKIN. Report of the British Association Committee on Electrical 

 Standards, edited by F. JEN KIN, p. 149. 



S 2 



