o:c 



llEPORT — 1876. 



from that for weaker ones, so that when the weak cnrrents were reversed 

 there would be no longer equilibrium. Since, in point of fact, the reversal 

 of the weaker currents did not affect the cqiiilibrium, it follows that the 

 bridge was in equilibruun for the weaker currents as well as for the stronger 

 ones, and therefore the conditions were the same for both, and Ohm's law 

 is true to within the limits of error of tlie experiment. The mode in 

 which the actual strength of the currents was measured and tlie limits of 

 error ascertained, are described in the following Report by Mr. Chrystal. 



Report on the Second Experiment. By G. Chrystai, 



As has been pointed out by Professor Maxvv'ell, the change in the specific 

 resistance of a linear conductor, if there be any such change owing to increase 

 or decrease of the current, will depend on the amount of current that passes 

 through unit of area of its section ; so that if C be the whole current passing, 

 r the specific resistance for infinitely small current, I the length, lu the section, 

 and h a constant depending on the nature of the conductor, then the resis- 

 tances of the conductor will be 



or if E be the resistance for infinitely small currents, llf 1 — 7t— ) *. 



It is clear, therefore, that by making up a resistance of very fine wire, say 

 ■^^ of an iuch in diameter, any such efiect as that we have been looking for 

 would be greatly multiplied. Accordingly the following experiment, the 

 principle of which is due to Professor Maxwell, was undertaken by the writer 

 of this Eeport. 



The figure represents a Wheatstone's bridge, in which the resistances AB 

 and BD are each equal to a (in the actual 

 experiment 30 ohms), AC a resistance made 

 up of a thin wire wliose resistance for in- 

 finitely small currents is E, (this we sui^pose 

 to be duly corrected for temperature, as will 

 be explained by-and-by), and partly of a 

 length of the thick platinum-iridium wire 

 of the B.A. bridge, whose resistance is ,r. 

 CD consists of a resistance composed of thick 

 wire equal to E, and of the rest of the 

 bridge-wire, whose resistance is I — x. 



With a current C, w being the section of 

 fine wire, its resistance is = E(l — /,C-), 

 where 



h 



" W 



If P=j{nr7j tc the approximate resistance of the whole bridge (we suppose 



that there is nearly a balance), B Ihat of the battery circuit; then E bcing- 

 the electromotive force of the battery. 



Fig. 5. 



C = 



E 



B-i-|o2E 



= PE. 



* The sign of h is chosen according to Schuster'^ 6ugg6stion. 



