54 REPORT — 1876. 



do ; and it is of course opposite in direction. If we make our alternations 

 quick enough the wire will not cool sensibly during the smaller current, nor 

 heat sensibly during the larger, but will settle down to a mean temjierature 

 between that due to the larger and smaller currents. 



In the above calculation we have supposed the resistance of the fine wire 

 for infinitely small currents to be that corresponding to this mean tempera- 

 ture, which will be constant throughout the experiment provided the electro- 

 motive forces do not vaiy. 



If, however, the alternations are not quick enough to ensure temperature- 

 equilibrium, then the thin wire will be hotter during the passage of the larger 

 current than it is during that of the smaller ; and there will be an effect 

 opposite to that we arc looking for, a result which appeared- in many of the 

 experiments. 



The experiment proved very difficult in practice, chiefly owing to the diffi- 

 culty experienced in getting a good alternator; and it was only after a great 

 many total or partial failures that any thing like success was attained. A 

 sketch of the progress of the experiment, with an account of the more im- 

 portant difficulties, and how they were finally avoided or overcome, may be of 

 some interest. 



In the fii'st place the galvanometer indications in a Wheatstone's bridge, 

 arranged as above described, are somewhat peculiar. 



Suppose we are somewhere near a balance for some temperature of the 

 thin Avire above that of the room ; then on turning on the current there is a 

 sharp kick in one dii-ection, say to the right, then a slower but still tolerably 

 quick swing over to the left, and then a gradual subsidence back to zero or 

 thereabouts, which may last for haK an hour or longer. If this were due 

 solely to variation in the resistance of the thin wire the curve of time-resis- 

 tance would be of this nature — 



Pig. G. 



It had been found that the thin wire was very sensitive to air-currents, 

 merely blowing towards it from a considerable distance sending the spot off 

 the galvanometer-scale ; in fact to get any approach to steadiness the wire 

 had to be enclosed in a box, and latterly it was enclosed in a narrow tube, 

 and that again loosely rolled in a silk pocket-handkerchief, and the whole 

 enclosed in a box. It was therefore at first suspected that the peculiarity in 

 question was due to air-currents ; but some experiments with the wire in an 

 exhausted tube showed that it was due to some other cause. This cause was 

 found in the slow heating of the thick wire against which the thin wire was 

 balanced; and some obvious experiments were made confirming this con- 

 clusion*. 



* The behaviour of the galvanometer is therefore explained in this way :— The first sharp 

 short lack ia due to the fact that before the thin wire is heated its resistance is much smaller 



