CONDUCTIVITIES OF METALS AND ALLOYS AT LOW TEMPERATURES. 385 
of flow of heat within the region of measurement, but they influence certain small 
corrections, to be considered presently. Before the rod was placed finally in the tube 
the leading wires from the coils were wedged in the holes at the base, and the 
distances apart of the three sleeves measured to 0 - 01 centim. The apparatus when 
fitted together was lowered into the Dewar tube, the connections made, and after 
a few minutes, during which the temperature throughout became uniform, the 
differences, if any, between the resistances of the platinum wires were measured. 
An electric current was then sent round the heating coil on the rod, and was 
adjusted so that a suitable difference of resistance between the upper and lower 
platinum coils was obtained. The heating current was then switched from rod to 
tube, and after 5 to 10 minutes the difference of resistance of the platinum coils, and 
the actual resistance of the lower coil, were found. 
If a second observation, taken 3 to 5 minutes later, gave a different value for the 
difference of resistance, a further interval of 3 to 5 minutes was allowed and the 
difference again tested, and this was repeated till the difference of resistance became 
constant. The current supplied to the heating coil and the E.M.F. at its terminals 
were then read, and the current switched from tube to rod. 
After 5 to 10 minutes the difference of resistance of the platinum coils was again 
found, and observations taken till it remained constant. The resistance of the lower 
coil was then measured, and the current in the heating coil and the E.M.F. at its 
terminals read; then the current was again switched from rod to tube and the 
measurements repeated. 
The mean of the two differences of resistance found when the heating current 
flowed round the tube, subtracted from the difference found when the heating current 
flowed round the rod, gives, so long as the rate of rise of temperature of the apparatus 
is regular, the difference of resistance which would be produced if the heating current 
were supplied to the rod and the temperature of the tube were kept constant (see 
p. 397). 
The Dewar tube was then filled with liquid air and, when the apparatus had 
cooled down to the temperature of liquid air, the remaining liquid was poured out by 
tilting the tube. 
Observations of difference of resistance between the platinum coils, with the 
heating current round the tube, were then made as before till the difference became 
steady, the resistance of one coil was determined, and the heating current and 
voltage measured. The current was then switched from tube to rod, and the 
observations repeated. In this way observations with the heating current alternately 
round rod and tube were made throughout the gradual rise of temperature of the 
apparatus from that of liquid air to some temperature above that of the air of the 
room, the rate of rise of temperature being, if necessary, increased by a constant 
electric current sent through the supplementary heating coil wound on the copper 
tube. 
3 D 
VOL. ccviii.—A. 
