THE THERMAL CONDUCTIVITY OF METALS. 173 
It was, however, found that the method of soldering just described, and used in 
this investigation, was very troublesome, and required some practice. If new 
apparatus were being made, it would certainly be much more convenient to make an 
alteration in this respect. 
The following plan would make the soldering of the wire a very easy process, 
instead of, as described, a difficult one. Six copper plugs, each of the shape shown 
in sectional elevation and plan in fig. 6, might be made, 8 miilims. in length, 

and 1 centim. diameter, so as to screw into a corresponding hole in the bottom of the 
heating box ; and six similar plugs might be made for the ball. In each of the plugs 
the central hole might be made of a different diameter, varying from 2 to 4 millims. 
In this way, wires of different diameters could very easily be soldered into the 
suitable plugs, which could then be screwed, first the one end into the ball, then the 
other end into the box. The two side holes shown in fig. 6 are for the purpose of 
facilitating the screwing in of the plugs. By screwing into the ball first, the possi- 
bility of twisting the wire is avoided. It is advisable not to use wire of diameter 
much less than 2 millims., for, in wires of less diameter, the length required to give a 
readable rise of temperature per minute becomes inconveniently small, unless the 
calorimeter ball is made very small. 
The calorimeter ball most frequently used in the present work was turned from 
the solid, and is 5°5 centims. in diameter. This was found to be a very convenient 
size for the metals of high conductivity—for example, gold, silver, and copper. 
Any length from 4 to 8 centims. of wires of these metals may be used without 
making it difficult to read the rise in temperature per minute. It is not advisable 
to use lengths shorter than 4 centims., as there is then a danger of the water- 
jacket being too near the heating-box, but it is better to use a smaller ball, say 
of 4 centims. diameter, for metals of lower conductivity, For metals having con- 
ductivities between 1:0 and 0°7 C.G.S. units, the large ball was found to be quite 
convenient ; for lower values the smaller ball was used. The object aimed at was 
to arrange the dimensions so as to enable an experiment to be finished in less than 
half an hour from the time that one end was made hot by the boiling water. 
After the wire has been soldered in, the box is placed in the screen L (fig. 2), and 
fixed by any convenient means, so that the wire hangs vertical, and the asbestos 
cover is put over the box. The cardboard tube, having been made of suitable 
length, and slit along its length, is now slipped round the wire, and, after a little 
cotton is loosely packed in, the slit is closed up by means of a strip of gummed paper. 
