AND RESISTANCE OF NICKEL WIRE AT HIGH TEMPERATURES. 41 
from the same piece giving data by means of which the measured resistance could be 
reduced to centigrade degrees. It is known™ that the resistance of nickel changes 
rather curiously with temperature, of which more hereafter. In the present case the 
resistance temperature graph is, roughly speaking, a long sloping S form of curve, and 
can be represented for interpolation purposes very approximately by three straight 
lines. Thus, to reduce the measured resistance r of the wire (3 ohms at 15°) to 
temperatures the formule are 
t 
t 
t 
56:77 — 163°3 from 15° to 200° 
30-97 — 9227 2005 ,, 350" 
95:87n— 6920) DOr 400° 
I i Wl 
2. THe Meruop or Hxprriment.—The resistance changes due to magnetization 
were measured by deflections on a delicate galvanometer after the bridge was approxi- 
mately balanced, a steady current being supplied from a secondary cell in the battery 
branch. The galvanometer was calibrated in the following simple way. For any 
particular condition the current 7 through the galvanometer is given by the formula 
Di=(lp- Maye . 3 oe ole: 
where e is the electromotive force in the circuit, and D the well-known expression 
involving the resistances of the six conductors making up the Wheatstone Bridge. Its 
value is 
D=BG(L+M+A+yp)+B(L+A)(M+p)4+G(L+ M)(A +») + LMA+4+p)+An(L4+ M), 
where B is the resistance in the battery branch and G that in the galvanometer branch. 
In the present experiments B was 6:2 ohms and G was 1°73 ohms. 
The calibration of the galvanometer in each experiment was effected by making a 
known small change in A; and the magnetic effect on resistance gave a slight change in 
L. We must therefore consider how the quantity D changes with slight changes of 
Aand L. Differentiation of D gives 
~ = BG+B(M+,) + (G+p)(L+M)+LM 
dD } 
ri ae BG+B(M+ ,) + (G+ M)(A+p) +Ap. 
The particular values of these quantities will vary from experiment to experiment 
according to the temperature of the branches L and M. For temperatures higher than 
the temperature of the air, L was always at a higher temperature than M, since it lay 
lower down in the porcelain vessel. ‘To obtain the approximate balance on the Wheat- 
stone Bridge « was appropriately varied so that the quantity (Lu— MA) rarely differed 
more than 1/400th from the value of Lu or of MA. When the difference was 1/200th the 
spot of light was driven off the galvanometer scale. In the following table the values 
* See my paper on the Electrical Resistance of Nickel at High Temperatures, Trans. R.S.H., 1886 ; also a paper 
by W. Koutravuscu, Wiedemann’s Annalen, vol. xxxiii., 1888. 
