Resistance according to an absolute Standard. 



265 



section, especially in fine wires, and hence, to obviate this diffi- 

 culty, Kirchhoff has indirectly ascertained the section of the wire 

 by determining its absolute and specific gravity. 



Now the determination of specific resistance according to this 

 unit, presupposes that the resistance of a wire whose length 

 remains unchanged, but the thickness of which is increased or 

 diminished, varies inversely as the section. This has not, how- 

 ever, been proved, and, with the small alterations of section 

 which are produced by pressure, can scarcely be proved. • There 

 is just as much reason for assuming that, if the mass and the 

 length of the wire remain unchanged, the resistance does not 

 alter even with a changing section. On this assumption the 

 absolute unit would have to be fixed in another way than as 

 being the specific resistance of a body whose absolute resistance 

 for the length = 1 and for a mass = 1 is equal to the fixed mea- 

 sure of resistance. According to this, the specific resistance of a 

 body would be determined by multiplying the resistance of a wire 

 formed of that substance expressed according to the fixed measure 

 of resistance by its mass, and dividing by the square of its length. 



The specific resistances of the wires used by Jacobi, Kirchhoff, 

 and myself will be determined according to the unit thus fixed • 

 for apart from the above considerations, this determination is in 

 any case the most applicable and capable of execution. 



The following Table exhibits the results of these determina- 

 tions : — 



Quality of copper in 



Length in 

 millime- 



Mass in milli- 

 gramn.es. 



Resistance in abso- 

 lute measure. 



Specific 

 resistance. 



Jacobi's wire ... 

 Kirchhoff's wire 

 Weber's wire ... 



7620 22435 

 355 4278 



394600(1 152890000 



5980000000 



5S500000 



190000000000 



2310000 

 1916000 

 1865600 



T&* 



It will be seen that there is only a small difference between 

 my copper and Kirchhoff s ; while the difference in the case of 

 Jacobi's is far more considerable, as the latter possesses a far 

 smaller conductibility. In the supposition that Jacobi may have 

 used galvanoplastic copper for his standard, 1 examined a wire 

 of that material which I procured through the kindness of Pro- 

 fessor Schellbach in Berlin, and found the following result, 

 which proves, contrary to the above supposition, that galvanoplas- 

 tic copper is a somewhat better conductor. 



Wire of galvano- 

 plastic copper. 



Length in 

 millime- 

 tres. 



Mass in milli- 

 grammes. 



Resistance in abso- 

 lute measure. 



Specific 

 resistance. 



- 



12780 



221295 



1243000000 



1684000 



T53- 



Phil. May. S. 4. Vol. 22. No. 147. Oct. 1861. 



