182 MR. J. H. GRAY ON A METHOD OF DETERMINING 

Mean conductivity between 
Length used (in centims.). 10° C. and 97° C 

7:0 867 
6:33 862 
5-7 | 859 
51 ‘858 



The greatest variation here is 1 per cent., and the mean value is ‘8612. 
Some specimens of copper wire were bought in a plumber’s shop, of quality used for 
bell-hanging. . 




| 
Speci Diameter Specific resistance | Mean thermal 
pel aes ted (in centims.). (electrical). | conductivity. 
1 -200 5545 “3198 
2 | 204 4701 3497 
Conductivity of Laboratory wire 9-78 Lee Bhat 
= =F or heat, 

Conductivity of Specimen 1 

= 2°86 for electricity. 
Conductivity of Laboratory wire : 
Conductivity of Specimen 2 AOE HOD INSEL 
= 2°56 for electricity. 
Conductivity of GLOVER’s wire 
: = 1°08 for heat, 

Conductivity of Laboratory wire 
1:066 for electricity. 
In all the wires tested it was found that if one metal was a better conductor for 
electricity it was also better for heat. This has been noticed by several investigators, 
notably Professor Tart, and WirpEMANN and Franz. Beyond this the present 
results cannot go, as enough trials were not made to allow of comparison. Previous 
experimenters have found that the ratio of electrical conductivities of two wires is 
not exactly equal to the ratio of thermal conductivities. This is indeed to be 
expected, if the coefficients already obtained for the alteration with temperature are 
accurate. For example, in copper the coefficient of variation per degree for electrical 
conductivity is very much greater than that found for thermal conductivity, so that, 
even if the ratios were equal at one temperature, they must be unequal at all other 
temperatures. In some of the wires tested the electrical and thermal ratios differed 
by as much as 4 or 5 per cent. 
Before the present investigation, the absolute values for the conductivity of the 
