122 REPORT—1890. 
of which an account is given in the same B.A. Report, and which were 
performed at a temperature of 20° C. 
I have, therefore, on this account, as well as for other reasons stated 
later, made my measurements at the temperature of the air, and believe 
that as his values were reduced by a temperature coefficient to values at 
0° C., I shall, by using the same temperature coefficient, obtain results 
directly comparable with my own measurements. 
For the measurement of the resistance of the specimens of wire a 
Wheatstone’s bridge arrangement was employed. Two of the arms of the 
bridge were formed by a 10 and 1 standard B.A. unit, namely, 66 and G; 
these were so nearly 10 to 1, that they were taken to be in that ratio. 
The third arm was 4 of a B.A. unit, and in the fourth arm 
was the wire to be measured; this was stretched on a flat board, and 
soldered at the ends to copper plates, to which connecting wires were 
also soldered; the length of wire used was generally a little less than 
two metres, and the wires were, approximately, No. 18 B.W.G. The 
board had scales screwed to it at the two ends. The board and wire were 
placed in a long bath made of zinc, and filled with paraffin. Wires which 
were left in the bath for some days, and, in more than one case, several 
weeks, were not found to have been acted on by the oil. 
One end of the wire, Py, Q2, was connected by a binding screw, through 
an adjustable resistance, 7 ($ metre of copper wire), to the mercury cup, 
@;, in which was one of the legs of the 4 coil, and also to a revers- 
ing key in the battery circuit. The 4and the 10 ohm coils were con- 
nected up together through an adjustable resistance, P, M,; one leg of 
each of the coils 10 and 1 was in the same mercury cup, L; and the 
other end of the 1 B.A. unit was connected with the other end of the 
wire, Py Qo. 
A single Leclanché cell was connected with the reversing key, and 
the fourth point of this key was connected with the mercury cup L, into 
which the legs of 10 and 1 dipped. In this circuit there was also a 
touch key. The galvanometer circuit was always made, and thus there 
was no thermo-electric effect in the galvanometer circuit. To each of 
the mercury cups Q;, P}, Mj, M2 were connected two thick wires with sepa- 
rate binding screws: one of these wires was welded to the copper plate 
at the bottom of the mercury cup. Each of these latter wires was con- 
nected with two way-keys; those in P, and Q, to the key k,; those in mM, 
to the key k, ; those in My to the key Ky. 
The base points of the keys kK, and ky were connected with a delicate 
reflecting galvanometer, that employed for the comparison of the stan- 
dards on the Fleming bridge. The base of the key k, was connected 
with the third point on the key xk, and the third point, on the key x), 
was connected to the base point of a fourth key, k., the two other points 
on this key being connected with riders, with which contact could be made 
with two points on the wire P, Q,; the riders had straight edges, and 
thus their position on the scales could be easily determined. In perform- 
ing an experiment, the keys K, and kK, were so connected that the mer- 
cury cups, and so the ends of the coils 10 and 1, were in circuit with the 
galvanometer. The resistance, P, M,, was then varied till, on making 
the battery circuit, no deflection resulted. The ends of the 10 and 1 
were then at the same potential, and as the other ends of these coils were 
connected with the same pole of the battery, there was the same fall of | 
potential on the two lines. 
