1 865 .] of Electrical Resistance. ' \ 55 



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 Fechnerf, and PouilletJ, 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 resistance of all parts of a 

 heterogeneous circuit into a given length of some given part of that circuit, 

 so as to form an imaginary homogeneous conductor, the idea of which lies 

 at the basis of Ohm's reasoning. 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 next step would naturally be, when comparing different circuits, 

 to reduce all resistances into a length of some one standard wire, though 

 this wire might not form part of all or of any of the circuits, and then 

 to treat the unit length of that standard wire as a unit of resistance. 

 Accordingly we find Lenz (in 1838) stating that 1 foot of No. 11 copper 

 wire is his unit of resistance, and that it is 19'9 times as great as the 

 unit he used in 1833*, which was a certain constant part of the old circuit. 

 In the earlier paper the resistances are treated as lengths, in the later as 

 so many "units." 



Lenz appears to have chosen his unit at random, and apparently with- 

 out the wish to impose that unit upon others. A further advance is seen 

 when Professor Wheatstone, in his well-known paper of 1843 1|, proposes 

 1 foot of copper wire, weighing 100 grains, not only as a unit, but as a 

 standard of resistance, chosen with reference to the standard weight and 

 length used in this country. To Professor Wheatstone also appears due 

 the credit of constructing (in 1840) the first instruments by which definite 

 multiples of the resistance-unit chosen might be added or subtracted at will 

 from the circuit || . He was closely followed by Poggeudorff^[ and Jacobi**, 

 the description of whose apparatus, indeed, precedes that of the Rheostat and 

 Resistance-coils, although the writer understands that they acknowledge 

 having cognizance of those inventions. Resistance-coils, as the means of 

 adding, not given lengths, but given graduated resistances to any circuit, are 

 now as necessary to the electrician as the balance to the chemist. 



In 1846 Hankelff used as unit of resistance a certain iron wire ; in 1847 

 I.E. Cooke $$ speaks of a length of wire of such section and conducting- 

 power as is best fitted for a standard of resistance. Buff and Horsford || ]| 

 * Pogg. Ann. vol. xxxiv. p. 418. 



t Maasbestimmungen, etc. 1 vol. 4to. Leipzic, 1831. 



J Elemens de Physique, p. 210, 5th edition ; and Cornptes Eendus, vol. iv. p. 207. 



Pogg. Ann. vol. xlv. p. 105. || Phil. Trans. 1843, vol. cxxxiii. p. 303. 



*[ Pogg. Ann. vol. Hi. p. 511. ** Pogg. Ann. vol. Hi. p. 526 ; vol. liv. p. 347. 



ft Pogg. Ann. vol. Ixix. p. 255. } J Phil. Mag. New Series, vol. xxx. p. 385. 



Pgg- Ann. vol. Lxxiii. p. 497. 



II II Pgg- Ann. vol. Ixx. p. 238, and Silliman's Journ. vol. v. p. 30. 



N 2 



