168 MR. J. H. GRAY ON A METHOD OF DETERMINING 
reaching the wire or the ball by the wooden screen L. In the bottom of the box, 
just above the Bunsen, are riveted a number of copper pins, so as to catch and 
distribute the heat of the burner. Fig. 4 shows a plan of the box. 

It will be convenient to classify the sources of error that may affect the results, 
and to deal with them at this point. 
1. There is, unless proper precautions be taken, loss of heat due to radiation from 
the surface of the wire, and therefore the value obtained for the conductivity will be 
too low. 
2. The temperature indicated by the thermometer in the hot water may not be the 
same as that of the end of the wire at the point where it enters the box. Also, the 
temperature of the ball may not be the same throughout. 
3. The thermometer may not indicate the average temperature throughout the ball. 
4, There may be a lag in the thermometer. 
5. There may be some error due to the solder at the ends of the wire. 
1. Loss of heat due to radiation from the surface of the wire. As this was the most 
obvious source of error, a long time was spent and much work done in investigating 1t. 
The cardboard tube G (fig. 2) was found to be thoroughly efficient in preventing 
radiation. It can be very easily made by rolling a long strip.of paper, which has 
been previously gummed on one side, several times round a circular rod of a centimetre 
diameter. The length of the tube is a little less than that of the wire. After the 
gum has dried, a slit sufficiently wide to admit the wire is cut parallel to the length. 
A circular piece of cardboard is fixed with gum to one end of the tube, as a flange for 
attaching the tube to the bottom of the heating box. After it has been arranged 
that the wire is in the central line, cotton-wool is put loosely in and the slit closed 
up with gummed paper. The small volume of air in this tube takes almost imme- 
diately the temperature of the wire all along its length. 
To test the efficiency of the tube, a large number of experiments was made, the 
results of which are given further on (pp. 180,181). It is clear that for a given differ- 
ence of temperature between the ends, the loss by radiation from the surface of a wire 
of given diameter will be greater the greater the length. If, then, there be no error 
which becomes less the greater the length, and thus compensates for the radiation error, 
a sufficient test of the efficiency of the tube will be to determine the conductivity with 
different lengths of the same wire. The shorter lengths ought to give a distinctly 
higher value, since the radiation from the surface is not only less than in the longer 
