CONDUCTIVITIES OF METALS AND ALLOYS AT LOW TEMPERATURES. 397 
Let v = v 0 + i\, where v 0 is independent of t, and v 0 + i\ 
conditions, 
S 2 * 
V the following 
0 = qk 
v n 
v 0 = 0 at x = 0, 
L 
r 
dv 0 
qk ~ +phv o = Q at x = x c , 
dx 
qcp d -£ = qk d ^-ph(v 1 -V), 
V = f(t), i\ = V at x = 0, > 
qk ~ +ph (v x — V) = 0 at x = x c , 
then v Q + i\ obviously satisfies the conditions laid down for v. The solution of the 
equations for iq is 
Hence v 0 = 0 when Q = 0. 
Since i\ is independent of Q, we have at any instant for all values of x 
■?’! with Q finite — v x with Q zero = v 0 . 
Since, however, v x cannot be observed at the same instant with Q zero and Q finite, 
temperatures are observed alternately with Q zero and Q finite, and the distribution 
with Q finite compared with the distribution with Q zero calculated for the same 
instant by interpolation between the observations with Q zero made immediately before 
and after. 
In all but three or four cases it was only necessary to take the mean of the 
observations with Q zero taken before and after. 
Since f(t) is a continuous function the error committed by this method of observation 
will be very small. 
Standardisation of Platinum Thermometers. 
As the observations of conductivity were made over the range of temperature 
between about — 180°C. and 30° C., the temperatures selected for standardising the 
thermometers used were the boiling point of liquid oxygen, the freezing point and the 
boiling point of water. 
The two thermometers were slipped on to a short rod of brass 2 centims. in length 
and of the same diameter as the rods used throughout the work. The rod and 
thermometers were then placed in a short brass tube about 3 centims. long, one end 
of which was closed by a brass plate. Through a cork in the other end of it the 
thermometer leads and a pair of compensating leads similar to those of the thermo- 
