ELECTRICAL APPARATUS. 167 



same principle, the force between the current and the magnet is 

 estimated from the amount of torsion of an. elastic fibre required 

 to balance it. 



Galvanometers, in which the effective distance of the conductor 

 from the magnet cannot be ascertained by direct measurement, 

 can be used for absolute measurements if the value of their indi- 

 cations has been determined by comparison with a galvanometer 

 of known dimensions. 



The strength of a current can also be deduced from the force 

 exerted between two measured lengths of the circuit, occupying 

 determinate relative positions. Instruments acting on this prin- 

 ciple are commonly called Electrodynamometers. The first was 

 constructed by Weber (1846). The law established by Fara- 

 day (1833), that, when an electric current traverses a compound 

 conducting liquid, chemical decomposition takes place in a quantity 

 of the liquid which is proportional to the quantity of electricity 

 transmitted, is the basis of the so-called voltametric system of mea- 

 suring electric currents. 



The strength of the electric current traversing a given conduc- 

 tor depends upon the material and dimensions of the conductor, 

 and upon the difference of potential between its extremities. 

 Stated generally 



the strength of an electric current in a conductor = difference of 

 potentials between its extremities x its conducting p 



* It is generally more convenient to express the effect of the dimensions 

 and material of a conductor upon the strength of a current conveyed by it by 

 speaking of its electrical " resistance," rather than of its " conducting power." 

 Conducting power is the reciprocal of resistance, and is used in the text for the 

 sake of bringing out more clearly the electrostatic analogy referred to in the 

 next line. The greater convenience of statements of resistance, as compared 

 with statements of conducting power, arises from the fact that, in practical 

 electrical problems, the addition and subtraction of resistances occurs oftener 

 than the addition and subtraction of conducting powers ; moreover, the term 

 conducting power seems to suggest that electrical conductors possess some 

 positive property, by virtue of which they are able to cause a transfer of 

 electricity, whereas the more correct conception probably is that they do not 

 possess any property which enables them to prevent the transfer. 



