Mr. F. Jenkin on the New Unit of Electrical Resistance. 477 



April 6. — Major- General Sabine, President, in the Chair. 



The following communication was read : — ] 



" Report on the New Unit of Electrical Resistance proposed and 

 issued by the Committee on Electrical Standards appointed in 1861 

 by the British Association." By Fleeming Jenkin, Esq. 



Sir Humphry Davy, in 1821*, published his researches proving 

 a difference in the conducting-power of metals and the decrease 

 of that power as their temperature rose. This quality of metals was 

 examined by Snow Harris, Gumming, and E. Becquerel, whose table 

 of conducting-powers, compiled by the aid of his differential galva- 

 nometer, and published in 1826f, is still frequently quoted, and is 

 indeed remarkable as the result of experiments made before the 

 publication by Ohm, in 1827 J, of the true mathematical theory of 

 the galvanic circuit. 



The idea of resistance as the property of a conductor was intro- 

 duced by Ohm, who conceived the force of the battery overcoming 

 the resistance of [the conductors and producing the current as a 

 result. Sir Humphry Davy, on the contrary, and other writers of 

 his time, conceived the voltaic battery rather as continually reprodu- 

 cing a charge, somewhat analogous to that of a Leyden jar, which 

 was discharged so soon as a conductor allowed the fluid to pass. The 

 idea of resistance is the necessary corollary of the conception of a 

 force doing some kind of work§, whereas the idea of conducting- 

 power is the result of an obvious analogy when electricity is con- 

 ceived as a fluid, or two fluids, allowed to pass in different quantities 

 through different wires from pole to pole. When submitted to 

 measurement, the qualities of conducting-power and resistance are 

 naturally expressed by reciprocal numbers, and the terms are used 

 in this sense in the early writings of Lenz (1833) |], who, with 

 Fechner^f, and Pouillet**, established the truth of Ohm's theory 

 shortly after the year 1830. 



The conception of a unit of resistance is implicitly contained in 

 the very expression of Ohm's law ; but the earlier writers seem to 

 have contented themselves with reducing by calculation the resist- 

 ance of all parts of a heterogeneous circuit into a given length of 

 some given part of that circuit, so as to form an imaginary homoge- 

 neous conductor, the idea of which lies at the basis of Ohm's reason- 

 ing. These writers, therefore, generally speak of the resistance as the 

 "reduced length" of the conductor, a term still much used in France 

 (vide Daguin, Jamin, Becquerel, De la Rive, and others). The 



* Phil. Trans. 1821, vol. cxi. p. 425. 



t Ann. de Chhn. et de Phys. vol. xxxii. 2nd series, p. 420. 



I Die galvanische Kette, mathematisch bearbeitet, 1827 ; also Taylor's Scien- 

 tific Memoirs, vol. ii. p. 401. 



§ The writer does not mean by this that electrical and mechanical resistance 

 are truly analogous, or that a current truly represents work. 



|| Pogg. Ann. yol. xxxiv. p. 418. 



•If Maasbestimmungen, etc. 1 vol. 4to. Leipzic, 1831. 



** Elemens de Physique, p. 210, 5th edition ; and Comptes Eendus, vol. iv. 

 p. 267. 



