for determining the conducting Power of Wires. 9 



that of what is called the galvanometer, is any certain measure 

 of the relative conducting power of a wire, without reference 

 to the intensity and productive power of the battery employed, 

 whether our inquiries relate to their length, diameters, or 

 natural qualities. To take the case of wires of the same kind 

 and same length, it may perhaps be possible, where the pro- 

 ductive power is great and the intensity inconsiderable, to em- 

 ploy a wire so small that it shall not be able to carry off the 

 whole of the fluid the battery is competent to supply; and when 

 this is the case a larger wire may be advantageously used, and 

 if it does not greatly exceed the other, the power of conduc- 

 tion and the indications of the galvanometer may be consistent; 

 but after employing a wire capable of conducting away the 

 fluid as fast as it can be supplied by the battery, it will be use- 

 less to expect to produce a greater effect on the galvanometer 

 by employing a greater wire. 



Should this view of the subject be correct, it must be ad- 

 mitted that the indications of the galvanometer are no certain 

 measures of the conducting power of wires, unless reference 

 be had also to the intensity and productive power. 



To take, by way of illustration, a more tangible, but some- 

 what analogous case, viz., the conducting power of metals for 

 caloric. I would suppose a spirit lamp of given power and in- 

 tensity to have one end of a wire placed in it, and the other 

 end in a vessel of water with a thermometer; there is, I con- 

 ceive, in this case no doubt that with small wires the ther- 

 mometer would indicate their relative conducting powers with 

 some degree of accuracy, but that it would fail entirely with 

 very large wires capable of conducting away the heat faster 

 than it can be generated. 



I may also observe that as some metals are known to be 

 better conductors of electricity than others, it is reasonable to 

 conclude that a smaller wire of a better conducting metal will 

 be as effective in carrying off the electricity of a battery as a 

 larger wire of a worse conductor, and hence, probably, the 

 discrepancies observed in the results of experiments on the 

 relative conducting power of different metals. Suppose, for 

 example, a copper wire to be employed, so small as not to be 

 able to carry off the supply of the battery, then the same-sized 

 silver wire will carry off more, and within certain limits the 

 galvanometer may indicate something like their comparative 

 powers of conduction; but if larger wires were employed a 

 different result might be obtained, the justness of the indication 

 depending, according to this view of the case, upon the cir- 

 cumstances whether both or either of the wires are larger than 

 is requisite to conduct away the supply. 



Third Series. Vol. 1 1. No. 64. July 1837. C 



