

478 Royal Society : — 



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 cir- 

 cuits, 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. 1 1 copper wire is his unit of resistance, and that 

 it is 19*9 times as great as the unit he used in 1833j% 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 

 without the wish to impose that unit upon others. A further advance 

 is seen when Professor Wheatstone, in his well-known paper of 

 1843 J, proposes I 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 Pro- 

 fessor 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 J. 

 He was closely followed by Poggendorff§ and Jacobi||, the descrip- 

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

 Resistance-coils, although the writer understands that they acknow- 

 ledge having cognizance of those inventions. Resistance-coils, as 

 the means of adding, not given lengths, but given graduated resist- 

 ances to any circuit, are now as necessary to the electrician as the 

 balance to the chemist. 



In 1846 Hankel^[ used as unit of resistance a certain iron wire; 

 in 1847 I. B. Cooke** speaks of a length of wire of such section 

 and conducting-power as is best fitted for a standard of resistance. 

 Buff ff and Horsford H in the same year reduce the resistance of 

 their experiments to lengths of a given German-silver wire, and as a 

 further definition they give its value as compared with pure silver. 

 To avoid the growing inconvenience of this multiplicity of standards, 

 Jacobi§§ (in 1848) sent to Poggendorff and others a certain copper 

 wire, since well known as Jacobi's standard, desiring that they would 

 take copies of it, so that all their results might be expressed in one 

 measure. He pointed out, with great justice, that mere definition 

 of the standard used, as a given length and weight of wire, was 

 insufhcient, and that good copies of a standard, even if chosen at 

 random, would be preferable to the reproduction in one laboratory of 

 a standard prepared and kept in another. The present Committee 

 fully indorse this view, although the definition of standards based 

 on weights and dimensions of given materials has since then gained 

 greatly in precision. 



Until about the year 1850 measurements of resistance were con- 

 fined, with few exceptions, to the laboratory ; but about that time 



* Pogg. Ann. vol. xlv. p. 105. f Pogg. Ann. vol. xxiv. p, 418. 



X Phil. Trans. 1843, vol. cxxxiii. p. 303. § Pogg. Ann. vol. Hi. p. 511. 



|| Pogg. Ann. vol. Hi. p. 526 ; vol. Hv. p. 347. ^[ Pogg. Ann. vol. lxix. p. 255. 

 ** Phil. Mag. New Series, vol. xxx. p. 385. ff Pogg. Ann. vol. lxxiii. p. 497. 

 • XX Pogg. Ann. vol. Ixx. p. 238, and Silliman's Journ. vol. v. p. 36. 

 §§ Comptes Eendus, 1851, vol. xxxiii. p. 277. 



