384 COMPARISON OF RESISTANCES. 



The two ends of a battery consisting of a great number of 

 identical elements arranged in series are closed by the two 

 resistances to be compared. Between the point of junction of the 

 two resistances, and a point of the battery, a bridge containing a 

 galvanometer is introduced, and the second end of the wire is 

 displaced until the needle of the galvanometer is brought to zero.* 

 Of the n couples which form the battery, the wire leaves / on one 

 side, and n -p on the other. As there is no current in the bridge, 

 the intensity I is the same in the two branches; if E is the electro- 

 motive force of an element, p its resistance, R and R' the two 

 resistances to be compared, which correspond respectively to the 

 numbers / and n p of elements, we have 



/E ( 



R+//O 

 from which 



R' n-p' 



983. When the resistance to be measured is that of the 

 insulating envelope of a cable, the current is very strong when 

 connection is made with the battery ; it then progressively diminishes. 

 The initial current is the superposition of three effects ; the charge of 

 the cable acting as condenser, the current which corresponds to the 

 phenomenon of electric absorption, and finally the leakage of the 

 cable. The first rapidly ceases, the second lasts a shorter or longer 

 time, the third alone is permanent; the resistance is deduced from 

 the intensity of this latter. 



The methods used in measuring resistance necessarily imply 

 the condition of a permanent state. Owing to the phenomena of 

 absorption and those of polarization, which are often met with in 

 the case of solids, this state is often only attained after the lapse 

 of some time. 



When the resistances are very great the final current is some- 

 times too weak to act upon a galvanometer. An electrometer is 

 then used. 



984. A very simple method consists in measuring the quantity 

 of electricity which the resistance in question lets pass, when a 

 constant difference of potential is established between its two ends 

 A and B. 



* FOUSSEREAU. Ann. de Chim. et de Phys. [6], Vol. v., p. 260. 



