494 
MR, C. H. LEES ON THE THERMAL CONDUCTIVITIES 
of wood, 30 cms. X 20 cms. base, and 15 cms. height. This was done in a drilling 
machine the table of which was found to be perpendicular to the spindle. One of the 
bars was then fixed by wedges into this hole in such a way that a point, which when 
the bar was in position would be the highest point of the cut surface, came in contact 
with a mark on the block of wood. The block and bar were then moved about on a 
stone and afterwards on a slate slab, till the surface was plane and smooth. The 
other bar was then fixed so that the lowest point of the cut surface came into contact 
with the same mark, and the surface rubbed down. This process secured that the 
cut surfaces should be parallel to each other when the bars were coaxal. To secure 
and test that the surfaces were parallel when the bars were in position in the 
framework, two test gauges were made, one for testing the upper surface and the 
other the side of the bars. The bars were taken out of the frame and placed vertical, 
the upper bar resting with its cut surface accurately on the cut surface of the lower 
bar, and not being supported by any other means. Four screws on each gauge were 
then adjusted so that they would touch four points, two on each bar near its 
extremities. By applying these gauges to the bars when in position, either in contact 
with each other or with a disc between, it is therefore easy to make the required 
adjustment of the cut surfaces. 
The surfaces were now amalgamated, the amalgamated surfaces brought together, 
and experiments made on the distribution of temperature throughout the bar, to test 
the efficiency of the mercury contact. When the bar was uncut, the difference of 
temperature observed between the points in the middle, 1 *05 cm. apart, was, as a mean 
of three experiments, '76° C. With the cut bar and the mercury contact, the distribution 
of temperature in the other parts of the bars being the same, the difference of tempera¬ 
ture observed was ‘04° C. The cutting and grinding of the bar had reduced the distance 
between the points of observation to '88 cm., hence if the mercury contact has no 
*76 x *88 
appreciable resistance the fall should be —— = '65°, which is, within the limits of 
error, what was observed. Hence, as far as the distribution of temperature along the 
bar is concerned, the mercury contact secures practical continuity. Occasional tests 
were made during the course of the work to see if this continued to hold. The 
resistance of the contact was found to increase slightly. Three tests made at the end 
of the work gave '78° as the difference of temperature. This is equivalent to an 
increase of distance between the points of observation from '88 cm. to 1'06 cm., and 
is probably caused by the gradual soaking of the mercury into the brass. Account 
is taken of this in the following calculations. 
The conditions of the above experiments are not quite the same as in experiments 
with crystals, &c., for while in the above there is only one mercury film, in the 
crystal experiments there are two. To investigate the effect of this, a thin film of 
mica, about '0003 cm. thick, was inserted. Although there are then two films 
of mercury and a film of mica between the bars, no change could be detected in the 
