658 Prof- Thdmaon on the Applications of Meclmmcal Effect 



which the strengths of the currents used arc detennined in ab- 

 •olute measure, the unit employed being the strength which a 

 current must have to decompose 9 grains of water in an hour of 

 time. But the electro-chemical equivalent of water, according 

 to the system of absolute measurement introduced by Weber, is, 

 in British units, very nearly -03, and therefore a current of unit 

 strength would decompose 72 grains of water in an hour. Hence 

 Joule's original unit is very exactly ^th of the British electro- 

 magnetic unit for measuring current electricity. By using the 

 formuUblmng nuot fii hfis / jjj -^ sib no eln^mn > 

 .. .^..lii R vd b3-iJj?.B9nT iaoTj^?=,-^*. 1 VfrfOTom ^Ai no x 

 d^Bs nr ^haB. ^-loionhno*) orlj jliiToult [)!?^ixf >(:// -lOj^^firon/r/to 

 and taking for 7 ©ne^eighth the number of Mr j Joule's "de- 

 grees of current/' for H the quantity of heat (measured by 

 grains of water raised 1° Cent.) evolved by the current through 

 the conductor experimented on; and for J the value 44758; I 

 have foundi loJonbiioi) i»jt^^ 13240000"^ M^^ '^^^ ^^^ ^^ tluj^ui 



-MM.Mii>j iJt 'i, .. .;..'i -i'li-nw, -idi')i\ ^i\Vi'i\ i\l 



as the absolute resistance of a certain wire used by Mr. Joule 

 for an absolute standard of resistance in the experiments on the 

 heat evolved in electrolysis, described in the second part of the 

 same paper*. ^ ; ■ i 



13. The '' specific resistance" of a metal referred to unity of 

 volume, may be defined as the absolute resistance of a unit 

 length of a conductor of unit section ; and the specific resistance 

 of a metal referred to unity of mass, or simply '' the specific re- 

 sistance of a metal" (since the term, which was introduced by 

 Weber, is, when unqualified, so used by him), is defined as the 

 absolute resistance of a conductor of uniform section, and of 

 unit length and unit weight. Hence, since the resistance of 

 conductors of similar substance are inversely proportional to 

 their sections, and directly proportional to their lengths, we have 



' km 



oil ^ = -j2> 



ifi; 'to dl/^. hill, i^i:. • )ii u1 li'itnV yi'u/ t. ito S- ? '■'■ 



'' * The 'iJir»i experiments £00111 ivvliioh the Aifmbwiil the text was de- 

 duced as a mean result (described in §§ 25, 26, 27 of the paper, Phil. Mag. 

 S. 3* vol. xix. p. 266), lead separately to the following values for the resist- 

 ^■IIOBp»**tJii on tjwiJ ,i97'jv^o:i 13260000 '^' Ji .iioirfi il'jiij n 

 .(MR-c/Don dlirr H^t'rn zhl "io 13360000 'f'"''J'>f'' ^^ .^^^■(^^ aV 



none of w1i\'ci['S¥*^'b'f ils'fatflcli as ^Vth from the mean givfeiiln'^Hetdit. 

 t By means of this I have found 4*1 for the specific resistance of cop- 

 per, according to the statement made in § 21 of Mr. Joule's paper, that 



