MEASUREMENT OF RESISTANCES IN ABSOLUTE VALUE. 



plane of symmetry, forms with the same needle a tangent compass 

 the constant of which, g' t is readily obtained with great accuracy. 

 The same current is passed through the galvanometer which 

 serves to measure the inducing current and through the frame 

 with a single wire. The deflections A and 6' give 



H h 



-tanA = tan 8, 



<* S 



and the expression for the resistance becomes 



Mg tan/? tan 8' 

 T g' tan A a 



1118. In the experiments in which this method was first applied, 

 Kirchhoff* used the same galvanometer for measuring the two quan- 

 ties I and m. The coils A and A' (Fig. 236) being placed in two 



Fig. 236. 



parts of the same circuit, one containing the battery E and the other 

 the galvanometer G, the resistance to be measured, forms a bridge PQ 

 between these two parts. The experiment consisted in measuring the 

 swing of the needle when the induced coil A was moved from the 

 position in which it was parallel to A', and for which the coefficient 

 of mutual induction is a maximum, to the perpendicular position in 

 which it is null. If r and r' are the resistances of the two circuits 

 A and A', E the electromotive force of the battery, and I and I' the 

 permanent intensities of the currents, we have 



* KIRCHHOFF. Pogg. Ann., Vol. LXXVI., p. 412. 1849. Gesamm. Abh. t \>. 118. 



