186 MR. J. H. GRAY ON THE THERMAL CONDUCTIVITY OF METALS. 
Fig. 7. 

The outside tube need not be more than 4 centims. diameter, and may be round, but 
the part where the wire is soldered in must be thickened and made as shown in the 
sketch, so that there may be no risk of water gathering above the wire. The length 
of the tube need not exceed 15 centims. This allows sufficient length for the tube to 
be inserted into the screen, which prevents the heat of the lamp from altering the 
temperature of the ball. The diameter of the inside tube may be 1°5 centims. With 
this apparatus most satisfactory results were obtained. 
When a metal is tested for the first time, it will be advisable to find its approxi- 
mate thermal conductivity by comparing its electrical conductivity with copper. This 
will give an idea as to which calorimeter ball to use, and what length of wire will: give 
a convenient rise of temperature per minute. A test is then almost as simple as for 
electrical conductivity. : 
