182. Dr. Harrison on Variation with Temperature of 
copper wire, wound in bobbins whose distance from the 
magnetometer is adjustable. The coils serve to compensate 
for direct action of the magnetizing and heating currents on 
the magnetometer. 
Magnetometer deflexions are read by scale and telescope 
arrangement. The scale was tested by comparison with a 
standard, but a calibration was found to be unnecessary. 
The magnetizing current (from accumulators) was generally 
read directly by means of a Hartmann & Braun ammeter 
calibrated against a standard Weston milliammeter. In some 
experiments, however, it was found more convenient to shunt 
part of the magnetizing current through the coil M (fig. 1) 
and to note the resulting magnetometer deflexion ; the value 
of 1 cm. scale-deflexion in terms of current was determined 
by previous experiment. 
By means of a liquid rheostat the magnetizing current 
could be continuously varied so as to produce inside the 
solenoid any field from zero up to 50 C.G.S. 
The value of the field controlling the magnetometer was 
obtained by sending a known current round a large copper 
ring, R, mounted as shown in fig. 1. From the deflexions 
produced the field could be calculated. 
The value of the field was tested frequently during each 
set of experiments. 
The Heating Apparatus.—The heating tube (pp, fig. 2) is 
of pure platinum, 30 cms. long, 2 mms. external diameter, 
and of 0:1 mm. wall-thickness. ‘The wire to be examined is 
placed symmetrically inside it. Satisfactorily to insulate 
the specimen from the platinum tube was found to be a 
matter of considerable difficulty. Mica disks were first tried, 
but they proved to be very unmanageable. Finally, small 
tubes of silica, each about 2 cms. long, were threaded on 
the specimen, which with this insulating sheath slid easily 
into the platinum tube. 
The tube is permanently soldered to hollow copper end- 
pieces EH’, and these screw into two copper rods OC’ (fig. 2) 
which form the heating-current leads. 
A pushing spring, P, of brass is arranged to take up the 
slack of the platinum tube when the latter expands, and the 
whole is inclosed in a glass tube which just fits the inner 
brass tube of the solenoid. 
The rods CC’ pass freely out of the glass tube, being 
supported by small brass tubes mounted centrally in rubber 
corks, one at each end of the glass. A clamping-screw, K, 
fixes C and enables the spring to be slightly compressed 
before the heating of the platinum is begun. 
