Method of Measuring Battery Resistance. 519 



observed, like that produced by an extra current, which is very 

 annoying. The modification which I have to propose, how- 

 ever, renders possible so great a virtual diminution of the 

 period of contact that this disappears. 



Modification of Mance^s Method. 



There is also a practical objection to the ordinary form of 

 Mance's method, not relating to its essentials, but to its sen- 

 sibility and convenience, which the modification is intended 

 entirely to remove. It is this : — The galvanometer in g, whose 

 function it is to indicate any change in the current in that 

 branch, has always a certain current passing through it, and 

 its needle is therefore deflected more or less, according to the 

 sensibility of the galvanometer; but the current produced by 

 an ordinary cell whose resistance one wishes to measure is 

 usually such as one does not care to pass through a delicate 

 instrument, even if the excessive deviation it produces be cor- 

 rected by external magnets. A rough galvanometer is there- 

 fore generally employed, and the needle is brought back 

 reasonably near its mean position by magnets placed near 

 it. But the needle being thus constrained by immersion in 

 a powerful magnetic field, is by no means under favourable 

 conditions, and only comparatively large changes in the cur- 

 rent can be indicated by it. To remedy this defect and to 

 make the method a null one, my first idea was to use a 

 differential galvanometer and to send through its second wire 

 a current from an auxiliary battery equal and opposite to 

 the current produced in its first wire by the cell whose re- 

 sistance is being determined, so as really to neutralize instead 

 of merely to overpower its action on the needle. Or, without 

 using a differential galvanometer, we may balance the elec- 

 tromotive force in the galvanometer circuit by means of an 

 auxiliary closed battery circuit after the manner of Poggen- 

 dorff. If either of these arrangements be adopted, we can 

 use a sensitive Thomson's galvanometer, and its needle may 

 be as nearly astatic as we choose. But it is not easy to get 

 the two batteries under such similar conditions that they shall 

 constantly oppose one another exactly ; and though these ar- 

 rangements may be useful in some cases, they are rather com- 

 plicated and the adjustments somewhat difficult to make. 



The next alteration which suggested itself consisted in inter- 

 posing a condenser in the galvanometer circuit (see fig. 3). 

 This effectually prevents any continuous circulation of electri- 

 city in that branch ; and the galvanometer therefore remains 

 at zero after the condenser has acquired its full charge ; but 

 any variation in this charge is indicated by a throw of the 



