372 



COMPARISON OF RESISTANCES. 



If the points A and B are displaced to A 1 and B x so that the 

 resistance A l B l is equal to AB, the intensity / does not change, and 

 the resistance of the portion AA X is the same as that of BB r And 

 in particular, if the point A l is come to the point B, the resistances 



Fig. 195- 



AAj and BBj are equal. This gives a method of dividing a given 

 resistance into two equal parts. 



Let PQ be the wire to be calibrated (Fig. 195), and AB the 

 portion which it is wished to divide in two equal parts at the point 

 M, A' and B' two points of any given wire P'Q'. Let i, 2, 3, 4, 5, 6, 

 be the cups of a commutator,* the connections of the different parts 



* This is a slight modification of PohVs Gyrotrope ( Kastner's Archiv., 

 Vol. xiii., p. 49, 1828), which is very frequently used as a commutator or 

 current reverser (Fig. 196). 



Fig. 196. 



The six cups A, B, A', B', P, and N are filled with mercury, and are 

 connected with the corresponding binding screws. The cups AA', BB' are 

 connected cross wise by two thick insulated copper wires. The two cups 



