482 
MR, C. H. LEES ON THE THERMAL CONDUCTIVITIES 
Outline of Method. 
The most important consideration in determining the method to be used is the fact 
that it is difficult to get large pieces of the crystals to be experimented on. This 
excludes methods requiring large plates, such as that of Weber aud Tuschmid, or 
large spheres or cubes, such as that of Kirchhoff or Thomson. A method which 
seemed to present several advantages was the one first suggested by Lodge/" and 
which may be called the “divided bar” method, and after some preliminary experi¬ 
ments had been made to determine its suitability, it was finally adopted. It consists 
in observing the temperature along a bar heated at one end and cooled at the other, 
and divided halfway between the two ends by a plane perpendicular to its axis, 
when (l) the divided ends are together, (2) a disc of the crystal or other body is 
between. 
If temperature observations are taken at several points iir each half of the bar, the 
corrections to be applied to the second set of observations for the distances of the 
points of observation from the contacts can be determined from the first set, and thus 
the temperature at each side of the crystal disc can be found. If, in addition, we 
know the thermal conductivity of the bar used, the amount of heat flowing through 
the disc can be found from the temperature slope ; and thence we have the thermal 
conductivity of the crystal. 
It may be noticed that there would be considerable uncertainty as to the nature of 
the contacts between bar and crystal unless some special precautions were taken. 
Lodge proposed to use pads of tinfoil to obviate this difficulty, but this method is 
not satisfactory. The difficulty has, however, been completely overcome by using 
bars of a material which would amalgamate, and making the contacts by means of 
mercury. 
Description of Apparatus. (Fig. 1.) 
The bar used in the experiments was one of brass, which presents several advantages. 
(1) It readily amalgamates, and therefore enables good contacts to be made. 
(2) Its conductivity is not so high as to make comparison between it and that of 
crystals, &c., impossible. 
(3) Its conductivity, according to Lorenz, increases with the temperature, a fact 
which, as will be seen later, partly neutralises the deviation of the cooling from 
Newton’s law. 
The diameter of the bar was l - 93 cm., and its length 67 cm. To each end of the 
bar was soldered a can, through one of these cans steam was sent and through the other 
water. The whole was arranged in a wooden frame, so that the bar was horizontal and 
exposed to the air over a covered tank through which a current of water could be 
sent to keep the temperature constant. The apparatus was surrounded by paper 
screens to protect the bar as far as possible from air currents in the room. 
* ‘ Phil. Mag.,’ (5), vol. 5, p. 110 (1878). 
