THE THERMAL CONDUCTIVITY OF METALS. 181 
prevent draughts, as the latter make a determination impossible. It was found that 
the paper tube could be shortened as much as to leave the lower half of the wire 
exposed without any appreciable diminution of the value obtained. When it was 
shortened much further, the value began to diminish till, when the tube was removed 
altogether, the value obtained was found to be fully 6 per cent. lower than when the 
tube completely covered the wire. When the wire was bare it was found very 
difficult to prevent draughts caused by the hot wire, and these draughts made the 
readings of the thermometer very irregular. 
In connection with this, it will be noted that there is a particular advantage 
in having the calorimeter ball lower in position than the heating box, as the hot air 
round the latter rises, and so does not affect the water-jacket. It has been suggested, 
however, that a separate experiment should be made to test whether any heat reached 
the ball otherwise than through the wire. For this purpose the ball was suspended 
by silk threads, in such a way that its highest point was 4°4 centimetres from 
the bottom of the heating box. The water jacket was placed round the ball, and the 
sheets of paper put on the top of the jacket. The water in the heating box was kept 
boiling for twenty-five minutes, and the temperature of the ball, as indicated by the 
thermometer in it, was noted at intervals. No change in the thermometer reading 
could be detected, although -45th of one degree can be read without difficulty on the 
thermometer scale. 
As a further test of the effect of the paper tube, the upper half length of the wire 
which was being tested, was left unprotected: by any tube, while the lower half was 
enclosed. The result was practically the same as if there had been no tube ai all. 
In order to make certain that the thermometer in the hot water indicated the 
temperature of the upper end of the wire, a thermo-electric junction was used. The 
junction was made of very thin wires of copper and platinoid, and, after being wrapped 
once round the hot end of the wire just at the end, was soldered there. The other 
junction was fixed to a thermometer and immersed in water. The heating box was 
then filled with water, which was kept boiling. The water in the vessel containing 
the other junction was gradually heated up till there was no deflection in the mirror 
galvanometer used. Both junctions must then be at the same temperature. When 
there was no deflection in the galvanometer the thermometers were found to be 
indicating the same temperatures. 
Several qualities of copper were tested, the results of which are given below. 
Copper wire, made by Messrs. Botton and Co., Cheadle. This wire was by 
mistake sent away before its electrical conductivity was measured, and it cannot be 
got again. 
