CONDUCTIVITIES OF METALS AND ALLOYS AT LOW TEMPERATURES. 403 
parts. This rod with its coils was suspended in the centre of the tube. A short 
brass rod 2 centims. long was fitted into tire base of the tube, and on the end 
projecting into the tube the lower temperature measuring coil was placed. The coils 
were connected up as in an experiment to determine conductivity. 
On sending a small current through the heating coil the temperature of the rod in 
the centre is raised, and when equilibrium between the electrical supply of heat and 
the loss of heat from the surface of the rod had been attained, the excess of tempera¬ 
ture of the rod over the temperature of the shorter one continuous with the enclosing 
tube was observed. The temperature of the latter and the supply of power to the 
central rod were also measured. To get rid of the effect of the slow change of 
temperature of the enclosure the heating current was then switched from the rod to 
the enclosure, and the difference of temperature between rod and enclosure again 
measured after the steady state had been attained. 
The same series of observations was made at about 100°, 200° and 300° absolute 
temperature. 
The total quantity of heat, H, imparted to the bar per second by the heating coil is 
given in gram-degrees by equation (17), p. 395, as 
H = 0-240GA {(1 — 0'057A) P 0 —(0 - 021 + 0 - 00011r) A} —0 - 0000048n c , —0 - 000013c B , 
where A = amperes sent through coil, 
P 0 = volts at ends of leads, 
r = absolute temperature of enclosure. 
v c , — v B = excess of temperature of rod over that of enclosure. 
If S is the total surface of rod and sleeves, and the coefficient h of loss from surfaces 
of rod and sleeve be assumed to be the same, since both were slightly oiled surfaces, 
we have 
H = S hv B . 
Hence 
hS = 0-2406A {(1-0-057A) P 0 -(0-021 + 0'00011t) A} 0'000018. 
Now 
S = 2 {(2-28 x 0-42) + (2T4 x 0*31)} for sides and ends of sleeves. 
+ (5-2 —U3) U84 + (2x 0-27) „ „ „ rod, 
= 10’96 sq. centims. 
v B = (1 -026 + 28-6/*,) v B (12), p. 392, and v B — 102-3 (R B —P T ) (19), p. 400. 
where v B is the mean temperature of the platinum temperature coil on the rod, Ii B its 
resistance, li T that of the coil in thermal contact with the enclosure. 
3 F 2 
