426 



EEPor.T — 1881. 



Appendix I. 



Account of Preliminary Experiments on the Determination of Electrical 

 Resistances in Ahsohite Measure. By Professor G. Caret Foster, F.E.S. 



The experiments to be described in wliat follows were made in the 

 Physical Laboratory of University College, London. The principle of the 

 method employed is essentially the same as that of the method long ago 

 pointed out by Sir William Thomson, and adopted by the first Committee 

 of this Association upon Electrical Standards in their ex^^eriments of 

 1863 and 1864, as well as by Lord Rayleigh in the repetition of these ex- 

 periments recently carried out by him in conjunction with Professor 

 Schuster. 



Every absolute measuremeiit of resistance is, by the natm-e of the 

 case, fundamentally the determination of the ratio of an electromotive 

 force to the strength of the current which it produces in the conductor 

 ■whose resistance is to be measured. In Sir William Thomson's method, 

 as is well known, the electromotive force is due to the action of the earth's 

 magnetism upon a coil of wire spinning about a vertical diameter, and its 

 numerical value is known from the rate at which the coil spins, the total 

 area enclosed by its windings, and the intensity of the horizontal com- 

 ponent of the earth's magnetism. The electromotive force generates a 

 current in the coil, the strength of which is known from the deflection of 

 a small magnet, hung at the centre of the coil, and from the intensity of 

 the earth's horizontal magnetic force. This last factor, entering similarly 

 into the expressions for the electromotive force and for the current, 

 disappears from their ratio, which gives in absolute measure the resistance 

 of the "wire forming the revolving coil. 



In the method now to be described there is again an electromotive 

 force generated in a revolving coil, just as in Sir William Thomson's 

 method, but the current is measured by an independent galvanometer, 



and the conductor, whose resistance 

 is given by the experiments, is en- 

 tirely distinct from the revolving coil. 

 So far as this method possesses any 

 particular advantages they arise frora 

 the circumstance last mentioned. The 

 conductor of which the resistance is 

 measured being at rest, it may be a 

 coil of wire of any material, wound 

 in whatever waj^ may bo most con- 

 venient ; and it may be immersed in 

 a bath of liquid so as to keep it at an 

 accurately known temperature. More- 

 over, several independent coils of 

 diiferent resistances can. be experi- 

 mented upon one after another, and 

 H[ \^ ^/ J the resistance of each determined. 



' ' The nature of the method and the 



arrangement of the essential parts of 

 the apparatus may be explained by help of the adjoining figure. In 

 this, R stands for the wire, of which the resistance is to be measured ; and 

 P for a thermopile whereby a current is produced through E and through a 

 tangent-galvanometer G. The ends of the wire of the revolving coil c are 



Fig. 1. 



