176 MR. J. H. GRAY ON A METHOD OF DETERMINING 
The numbers in column I. are put on a curve, as shown in diagram II. From this 
curve the rise in temperature in half a minute is read off for «° above the temperature 
of the air, and this is added to the rise for «° below the temperature of the air, where 
« is any value from 0 to 5 or 6. In this way as many as 10 or 15 values are 
obtained. If the curve were quite regular, each of these values would be the same, 
since they each represent the rise in temperature per minute for the same difference 
of temperature, but they are found to vary by about 1 or 2 per cent. But, by taking 
the mean of 10 or 15 values, it will be seen that the result obtained must be very 
near the correct value for the rise between the given temperatures. It is then only 
necessary to multiply this rise in temperature by the thermal capacity, C, of the ball 
to find the quantity of heat that has passed along the wire in one minute, and the 
conductivity can be calculated from the formula 
s Cal 
~ ar? (T —#) 60 
where 7 is the radius of the wire. 
In order to obtain an accurate determination of the thermal capacity of the ball, I 
took it to Dublin, where the capacity was most carefully determined for me, during 
the time I was there, by Dr. Joun Joty, F.R.S., by his most ingenious steam 
calorimeter method.* I have to thank Dr. Joty for the trouble to which he put 
himself in making the determination. 
As the most important thing in testing the method is to show that with different 
lengths the resulting determination of the conductivity remains practically the same, 
these series of tests and the results will be given first. 
The wire first used was what was called six years ago high electrical conductivity 
copper. The diameter was 2°1 millims., density 8°85, volume specific (electrical) 
resistance, 1834 in electromagnetic units. This wire was almost exclusively used, 
but in the course of the work tests were made of wire got from Messrs. GLOVER and 
Co. in the end of the year 1890. Messrs. Glover’s wire was found to have con- 
siderably higher conductivity, both electrically and thermally, than the first- 
mentioned wire, which for convenience will be called the laboratory copper, as it was 
what was used for all the electrical work in the laboratory. Taking the laboratory 
wire as 100 per cent. conductivity (thermally and electrically), it was found that 
Messrs. GLOVER’S wire was 106°6 per cent. electrical, and 108 per cent. thermal con- 
ductivity. 
These results will be referred to further on (p. 180). They are merely mentioned 
here to show that the best conducting wire was not used for the exhaustive tests. 
Table II. shows the record of series of experiments made on laboratory copper wire. 
The wire was soldered into the heating box and ball as described ; an experiment 
was made and the length measured carefully. The ball was then heated by a blow- 
* J. Jony, ‘Roy. Soe. Proc.,’ vol. 41, p. 352, 1886, 
