630 Dr. E. P. Harrison on the Zemperature-Variation 
same current (fig. 2). The standard was made of manganin 
wire, and was kept cool by being immersed in paraftin- oil 
and surrounded by a coil of “composition ” tubing through 
which cold water circulated. At two points on the 1 manganin 
were soldered fine copper wires, which, like the pair of 
platinum wires coming from the nickel, served as permanent 
‘* potential-leads.” 
The resistances a these two pairs of potential-leads were 
adjusted to within 31, of an ohm of one another by means of a 
small coil of fine german-silver wire in series with one of the 
copper leads (from the manganin). The resistance of the 
manganin wire between the points of attachment of the two 
leads was carefully deter mined, and will be referred to as 
“the standard resistance.” 
The galvanometer-circuit consisted of a high-resistance 
D’Arsonval in series with about 10,000”, and the deflexions 
of the mirror were read by scale and telescope in the usual 
way. The scale was by Casella on porcelain, and calibration 
was considered to be unnecessary. 
In making an observation of resistance, the pair of potential- 
leads from the nickel, and the pair from the standard, were 
alternately Be to the terminals of the oalvanometer- 
circuit G (fig. 2), corresponding deflexions of the galvanometer 
being noted. These deflexions are proportional to the resist- 
ances of the nickel and standard respectively, provided that 
the resistances of the two pairs of potential-leads are approxi- 
mately equal. The accuracy of the resistance measurements 
under these conditions is as great as that of which the particular 
scale and telescope arrangement was capable*. It was arranged 
that deflexions due to the nickel were as nearly as possible 
equal to those due to the standard. A special mercury switch 
was constructed for the purpose of quickly putting nickel or 
standard potential-leads into the galvanometer-circuit. 
The temperature of the nickel was deduced from a curve 
previously obtained which gave the relation between the 
resistance of a certain length of the wire and its temperature. 
If “” is the resistance of the present wire (A) at, say, 18° 
and R its resistance at 7°; 
And if ‘‘7”’ is the resistance at 18° of the wire (B) whose 
temperature coefficient is known ; 
And if § is the resistance of the “standard ;” 
Then we have 
ok 
B * 7 
Resistance at ¢° of wire B= Peas 
* If the two pairs of potential-leads differ ne ‘2 ohm, the error in Ni 
resistance is 2X10—-7 uhm; and this would give rise to an error of 
2x10—-6 ampere in the current through the ealy anometer—an estimate 
which is outside the figure of merit of “the instrument used. 
