PROFESSOR TAIT ON THERMAL AND ELECTRIC CONDUCTIVITY. 739 
For German silver (mean of several experiments at different parts of the 
bar, with correction 0:2 inch),— 
Ratio. 
G. S. C. Uncorrected. Corrected. 
10 84:1 8:41 8:56 
For lead (also with the correction 0:2 inch),— 
Ratio. 
Pb. C. Uncorrected. Corrected. 
14 93°7 6°69 6°77 
10 . 66°9 6:69 6°80 
These experiments were repeated for me by Mr D'Arcy Tuompson, who 
used, as contact pieces, plates of copper pressed edgeways against the long bars 
in planes perpendicular to their axes. THis results differ in no case from mine 
before the third significant figure. 
§ 23. Taking the inferior copper as unit, both for thermal and for electric 
conductivity, we find the following table of conductivities at ordinary tempera- 
tures, with the rough results as to specific gravity and specific heat referred to 
in § 15 above :-— 
Thermal. Electric. 
Copper (Crown), . : : , ‘ 1-41 1729 
Be pe « ‘ : , 2 ; 1:00 1:000 
Forses’ Iron, : : : : 3 0:29 0:264 
Lead, ; : : ‘ : : 0-12 0149 
German Silver, . : ; : ; 0714 0117 
The agreement of these numbers is by no means so close as is generally 
stated ; but this is no longer remarkable, for it is well-known that the electric 
conductivity of all pure metals alters very much with the temperature, while 
we have seen that, as regards thermal conductivity, there is but slight change 
with either copper or lead, though there is a large change with iron. This 
accords with some results of my own on the electric conductivity of iron at 
high temperatures (Proc. R. S. E., 1872-3, p. 32), and with the results of the 
repetition of these experiments by a party of my laboratory students (Proc. 
R. S. E., 1875-6, p. 629). 
The only alloy treated above, violates, as was to be expected, Forses’ rule 
for pure metals, for it seems to be superior to lead in thermal conductivity, 
while decidedly inferior to it as regards electric conductivity. 
§ 24. The chief results of these papers may be thus briefly summarised :— 
1. The thermal conductivity of tron diminishes as its temperature ts raised. 
This accords with the statement of ForBes, whose numbers for temperatures 
between 50° and 150° C. are probably very accurate. 
