1884.] of Conductors through which a Current is passing. 179 



currents of air, and from radiation of the sun or laboratory fire, and 

 from the persons of the observers. 



To commence the experiment a very feeble current was passed 

 through the line consisting of the copper and the German silver wire, 

 and by means of a sensitive reflecting galvanometer of high resistance 

 the differences of potentials at the two ends of AB, the copper wire, 

 and of BC, the German silver wire, were determined (fig. 1). The ratio 

 between these differences of potentials is the same as the ratio between 

 the resistances of AB and BC. Then, BC, the standard of German 

 silver, being known, the resistance of AB was calculated. This 

 experiment, therefore, gave the resistance of AB cold. 



A much more powerful current was then caused to flow through 

 the line, and was kept up for a sufficient time until AB had taken a 

 permanent temperature. The terminals of the potential galvano- 

 meter were again applied, and the differences of potentials at the 

 extremities of AB and of BC once more determined, and thus the 

 ratio of their resistances. If the resistance of BC had remained 

 absolutely constant, the resistance of AB hot from the effect of a 

 current, the magnitude of which is shown by the ampere-meter, would 

 now be known. This was practically the case. The quantity of oil 

 surrounding the German silver wire was so considerable that the 

 temperature rose but little, and the change of resistance of German 

 silver with temperature is so small that it was unnecessary to apply 

 any correction. It would be easy to keep the temperature of the oil 

 quite constant by occasionally passing a test tube containing a little 

 ice or a small freezing mixture from place to place through it, and 

 then stirring the whole up thoroughly. 



The resistance of the copper wire first cold and afterwards hot having 

 been determined, and the temperature of the wire when cold being 

 known (as at the beginning the temperature of the wire was the same 

 as that of the air), it was easy to find the temperature of the wire 

 hot by applying the formula for temperature correction for the 

 resistance of copper. 



I have also used, latterly, for passing a powerful current through 

 a wire under experiment, and for determining the resistance of the 

 wire while the current is flowing, a "Wheatstone's bridge, suitably 

 arranged for the purpose, in accordance with a plan suggested by 

 Sir William Thomson, and I find this method more sensitive and 

 more generally convenient than the potential method. 



The diagram (tig. 2) explains the arrangement. ABC and A'B'C 

 are the two branches of the Wheatstone's bridge, and the four con- 

 ductors are made up as follows : BC is the wire under experiment as 

 to heating by the current ; linn and op are two bars of very stout 

 copper, and between op and the portion Im of the bar Imn there are 

 soldered a considerable number of stout copper wires making up a 



N 2 



