
of the Coefficient of Expansion of Pure Nickel. 631 
also R _ deflexion due to nickel _ Oni, 
S  deflexion due to standard 6g, ’ 

whence 
/ 
e . Y he é = 
Resistance at ¢° of wire B=--S ™! 
Nf. Ost 

; » Oni 
=(in the actual experiment) *2346 a 
St 
Thus in order to determine the temperature of the wire 
under experiment it was only necessary to read off the 
abscissa (¢°) corresponding to the resistance as calculated 
above. 
Method of Observation. 
Hach of the two microscopes having been focussed on the 
nickel, so that the cross-wire lay on an edge of the appropriate 
scratch, readings of the microscopes were taken when the 
wire was at the temperature of the laboratory. The value of 
this temperature was obtained in two ways: firstly, by means 
of a mercury thermometer laid along the groove; and, secondly, 
by passing through the heating circuit a current, so small as 
not appreciably to heat the wire, but by means of which the 
resistance of the nickel and therefore its temperature could 
be determined. The two temperatures agreed very well, and 
served to check the adjustments. 
The heating current, obtained from storage-cells, was then 
started, and when the temperature was steady as observed 
by the galvanometer-reading, observations of deflexion were 
made, and immediately afterwards the micrometer-heads of 
the microscopes were turned till the cross-wires again lay on 
the edges of the scratches. Deflexions due to nickel and 
standard were once more noted, and the micrometers read. 
The heating current was then increased, and the process 
repeated at temperature intervals of 15° or 20° till the nickel 
beganto glow. The same operations were then gone through 
while the temperature of the wire was decreased. 
The heating current varied from about 2 amperes to 
10 amperes, at an E.M.F. of 26 volts. 
Discussion of Results. 
In the curve (fig. 3) is exhibited the actual change of length 
as the temperature varies between 0° and 500°. If “L”’ is 
the distance between the scratches at any particular tempe- 
perature, the ordinates on the curve show the values of 
(L—10) x10? cms. Up to a temperature of about 365° the 
