80 Mr. H. Tomlinson on the Effect of Heating and 



ance of the coil at 17° C. and its temperature-coefficient were 

 known, the resistance of y at any other temperature could be 

 calculated. 



It is usual in experiments with the " bridge " to close first 

 the battery- circuit, and immediately afterwards that of the 

 galvanometer ; but the author prefers to keep the galvano- 

 meter-circuit always closed, and to observe the effect of closing 

 the battery-circuit. It is true that in the latter case there is 

 always a momentary throw of the needle due to self-induction 

 in the wires, but this, with the arrangement shown above, is 

 very slight ; and with no more delay than that of two or 

 three seconds, it can be easily ascertained whether the act of 

 closing the battery-circuit causes any alteration in the difference 

 of potential at the galvanometer-terminals*. If, on the con- 

 trary, the former of the two methods be employed, there 

 arises more or less inaccuracy from the presence of thermo- 

 electrical currents set up in one or more branches of the bridge 

 by slight variations in the temperature of the air at different 

 parts of the room. Besides, even if the air could be main- 

 tained at perfectly uniform temperature throughout, thermo- 

 electrical currents are always produced by the act of pressing 

 down the sliding-piece, S, on the wire ab, and by Peltier 

 effects. 



The act of bending the wire into the form of the coil .pro- 

 duced, as might be expected, a slight permanent change in 

 its resistance, which was thereby increased by \ per cent. ; 

 doubtless a portion, but only a small one, of the whole reduc- 

 tion of resistance, to be presently recorded, is to be attributed 

 to the partial or complete removal of the effect of coiling. 



When everything was ready, and the wire had been left 

 undisturbed for several hours, its resistance and temperature 

 were determined ; the latter was then raised to 100° C, or 

 very nearly to 100° C, and maintained thereat for at least 8 

 hours f ; and during this period the resistance of the wire was 

 tested several times. The wire was then permitted to cool 

 down, and its resistance was again determined about 16 hours 

 afterwards. The same operations were repeated again and 

 again, until the metal showed that no sensible change of re- 

 sistance was produced by the heating and cooling. The results 

 are given in the following Table : — 



* There is always a difference of potential at the galvanometer- 

 terminals arising from therm o-electrical effects ; see what follows. 



+ The deviation from 100° C. never exceeded T 3 „ C, and could in all 

 cases be accounted for by changes in the barometric pressure. 



