DETERMINING THE CONSTANTS OF A VOLTAIC CIRCUIT. 
323 
conducting 1 wire*. The distance between these points must remain the same in all 
comparative experiments, but the absolute deviations of the needle will be greater as 
these points are further from each other. In the case of the circuit of a powerful 
electro-magnetic engine, or of a volta-typing apparatus, the diminution of resistance 
occasioned bv connecting the galvanometer wire in the manner above described is so 
trifling that it would be useless to take it into account, and the compensation above 
alluded to is, therefore, unnecessary. 
§ 16. The Differential Resistance Measurer. 
The method of determining the resistance of metal wires and other conductors of 
electricity by means of the rheostat, described in § 9, is inapplicable when small 
differences are to be observed. If, for instance, a short length of wire has to be ex- 
amined, its resistance is so small compared with the other resistances in the circuit, 
including that of the battery, that whether it be interposed or not, no change is ob- 
servable in the deviation of the needle ; and, even if greater lengths of the conducting 
substance be employed, fluctuations in the power of the battery frequently render 
the observation uncertain. 
The differential galvanometer proposed by M. Becquerel, had it been an instru- 
ment as practically as it is theoretically perfect, would have enabled us to ascertain 
very minute differences of resistance with great facility. But it is almost impossible 
so to arrange the two coils that currents of equal energy circulating through them 
shall produce equal deviations of the needle in opposite directions, the consequence 
of which is that the standing of the needle at zero is no indication of equality in the 
currents. This and other defects have prevented the differential galvanometer from 
coming into use. 
All the advantages, however, which were expected from this instrument may be 
obtained, without any of its accompanying defects, by means of the simple arrange- 
ment I am about to describe, which, moreover, has the advantage of being immedi- 
ately applicable to any galvanometer, instead of requiring, as in the former case, the 
instrument to be peculiarly constructed. 
Fig. 5 represents a board on which are placed four copper wires, Z h, Z a, C a, C b, 
the extremities of which are fixed to brass binding screws. The binding screws Z, C 
are for the purpose of receiving wires proceeding from the two poles of a rheomotor, 
and those marked a, b are for holding the ends of the wire of a galvanometer. By this 
arrangement a wire from each pole of the rheomotor proceeds to each end of the gal- 
* Professor Petrina of Linz has proposed (Poggendorff’s ‘ Annalen,’ vol. lxii. 1842, No. 9) a similar means 
of measuring and comparing electric currents of every degree of force. He interposes in the circuit a canal of 
mercury, the section of which is four square lines, and plunges into it, at various distances from each other, 
the ends of the wire of a sensitive galvanometer. He shows that if the resistance in the galvanometer wire be 
very considerable, and that of the mercury in the canal be small in comparison, the force acting on the galva- 
nometer needle will be sensibly proportional to the distance between the ends of the wire, and he has founded 
on this principle a ready approximative method of graduating the galvanometer. 
2 u 
MDCCCXLIIl. 
