CONDUCTIVITIES OF METALS AND ALLOYS AT LOW TEMPERATURES. 427 
metal there is a hard and a soft state, with different physical properties. If in the 
future a still closer agreement between the results obtained by different observers is 
demanded, it will be necessary to specify both chemical composition and microscopic 
structure of the materials tested. 
The fact that the thermal conductivities of most of the pure metals decrease with 
increase of temperature, while those of all the alloys tested increase with increase of 
temperature, raises the question whether the increase with temperature observed in 
the case of aluminium both in Jager and Diesselhorst’s experiments and in the 
present ones, and in that of nickel in the present experiment, cannot be attributed to 
small amounts (less than 1 per cent.) of impurity remaining in the specimens tested. 
While there is much to be said in favour of this in the two cases mentioned, it 
seems to me that the apparent existence of a maximum of conductivity of silver, and 
possibly of steel, and a minimum conductivity of aluminium, shows that we must 
guard ourselves against stating that the thermal conductivities of all pure metals 
decrease as the temperature rises, and limit ourselves to the less general statement 
that the thermal conductivities of most pure metals decrease with rise of temperature 
over the range —170° C. to +100° C., while those of all the alloys tested increase 
over the same range of temperature. 
On comparing these results with those obtained previously for non-metallic sub¬ 
stances,* mostly electrical insulators, it will be seen that on the whole metals and 
non-metals are affected in the same way by change of temperature, i. e ., both tend to 
conduct heat better at low than at high temperatures, while alloys conduct better at 
high than at low temperatures, and in this respect resemble glass, the only mixture 
of non-conductors of electricity which has up to the present been investigated. 
Section 2.—The Electrical Conductivity Measurements. 
The effect of temperature on the electrical resistivity of the material of each rod 
was determined by sending an electric current along the rod and through a standard 
small resistance in series with it. The rod was supported on a pair of knife edges, 
4 centims. apart, and the difference of potential between 
the two was compared with that between the potential 
terminals of the standard resistance by the method 
shown in fio-. 4. 
© 
a is the rod, b, c two short copper cylinders provided 
with slightly conical holes into which the ends of the 
rod were gently forced, d, e are the knife edges which 
support the rod and serve as potential terminals, f is 
the standard resistance of (P000994 or CL0001182 ohm. The current was supplied 
from a storage cell g, and could be adjusted by means of the resistance h. 
* Lees, ‘ Phil. Trans. Roy. Soc.,’ A, 191, p. 399 (1898); A, 204, p. 433 (1905). 
3 I 2 
