OF CRYSTALS AND OTHER RAD CONDUCTORS. 
495 
difference of temperature of the observing points. The same held on insertion of a 
piece of platinum foil '0025 cm. thick. The corrections for the short length of bar 
and mercury contact between the points at which the temperatures are observed and 
the surfaces of the discs, can, therefore, be determined from the observation with the 
amalgamated ends of the bars in contact. 
In preparing the apparatus for an experiment, the bars and framework were 
placed so that the bars were vertical, with the cool bar at the top. The bars were 
taken out of the frame and the holes and contacts cleaned with dilute nitric acid, 
provided with mercury, washed, and then dried by filter paper. The hot bar was 
then inserted into lower part of the frame, and if a disc was to be experimented on, 
it was placed on the mercury surface and the excess of mercury forced from under it. 
In the case of a transparent disc it was possible to see that the mercury formed a 
perfect mirror. The cool bar was then put into the upper part of the frame, and held 
up, in contact with the centre screw at the top of the frame, by means of a spring. 
The amalgamated end would be thus brought about ’5 cm. above the upper surface 
of the disc. It was then supplied with mercury, which hung down as a pendent drop. 
The upper screw was then turned till the end of the upper bar was about '5 mm. 
from the upper surface of the disc. In this position the gauges were applied to the 
bars, and the loops so adjusted by means of the screws at their ends, that the four 
points of each gauge touched the bars. The upper screw was then rotated till the 
upper bar came into contact with the disc, and the excess of mercury was forced out. 
The frame was then placed over the water tank with the bars horizontal and con¬ 
nections made to the steam and water supplies. The apparatus was allowed to stand 
about one and a half hours, in order that the distribution of temperature might become 
steady before observations were taken. Almost invariably on taking the bars apart 
at the conclusion of an experiment, the discs were found to adhere to the cool bar, 
and in the case of transparent discs this enabled it to be seen if the contact between 
the cool bar and the disc had been good. It was generally found as good a mirror 
as the contact between the hot bar and the disc. Some experiments were however 
made to try the effect of making contacts which were poor as far as could be judged 
by the appearance of the mirrors. No difference could be detected in the observations 
between an optically good and a bad mirror, so that a few experiments, in which the 
contacts were optically defective, have been included in the results given. 
Reduction of Observations with Discs. 
We have seen that the temperature throughout any short length of the bar may 
be represented by the empirical equation v = A cosh ax -j- B sink ax. We assume 
that this equation holds for the first three observations on each side of the disc, and 
thus have, if x v x. 2 , x 3 are the coordinates of the points of observation, measured from 
the surface of the disc, and v lf v 2 , % the observed temperatures, 
