256 MEASUREMENT OF CURRENTS. 



resistance. Hence, if the distance AB of the points of contact are 

 measured, the deflections observed for different values of this dis- 

 tance with the same current give the table of graduation. 



874. The most rapid method is to utilise Ohm's law by varying 

 according to a known law the intensity I of the current produced 

 by a constant electromotive force, such as a Daniell's cell, by the 

 addition of graduated resistances. If the internal resistance of the 

 couple is very small compared with the total resistance, the various 



intensities I, I', I" of the current, which corresponds to the 



resistances R, R', R" of the portion of the current external to the 

 galvanometer, give the ratios 







i i 



If the resistances R, R', R" are themselves very great compared 

 with the galvanometer, the equations reduce to 



R R 7 R/ 7 



The experiment is very simple when the galvanometer is pro- 

 vided with a shunt well adjusted with a compensating resistance. 

 The shunt being placed to the galvanometer, the external resistance 

 is regulated so that a deflection of n divisions is obtained. The 

 shunt is removed. The principal current is not altered, but it 

 becomes m times as great in the galvanometer, and gives a fresh 

 deflection n'. If we take for m a number which is not very high 

 (2, for instance), a table is readily constructed for the instrument 

 by varying the value of n. 



Several other methods have been proposed with the same object ; 

 but we shall not dwell on this question, which has lost most of its 

 interest. 



875. COMPARISON OF GALVANOMETERS. Let us assume that 

 for two galvanometers G and G' the constants of the coils are 

 supposed constant ; let H and H' be the horizontal intensities of 

 the external field on each of the needles, 8 and 8' the deflections 



