THE THERMAL CONDUCTIVITY OF METALS. 169 
lengths, but the quantity of heat conducted along the wire per second is greater, and 
therefore the percentage error due to radiation rapidly diminishes. 
In order to determine a superior limit for the error due to loss of heat by radiation, 
a calculation is made below. The numbers used are taken so as to be as unfavourable 
as possible, and give a value much higher than what would practically turn out. 
in the case of copper wire, the surface was always somewhat tarnished, so the 
emissivity may be taken as intermediate between those of polished and blackened 
copper, that is ‘0008. The diameter aud length of wire were respectively ‘2 centim. 
and 4 centims. The temperature at the hot end was 98° C., at the cold end 14° C. 
The curve of temperature along the wire being logarithmic, the mean temperature 
will be considerably less than half the sum of the highest and lowest temperatures, 
which would assume that the curve is a straight line passing through the highest and 
lowest points. Take, however, the curve to be a straight line; then the mean tempe- 
rature above that of the air is 42°C. The quantity of heat Q, lost from the surface 
S, of emissivity E, in one minute, is 
Q = ES x 42 x 60 
= 1:91 C.G.S. units approximately. 
The quantity of heat conducted along the wire during this minute, as taken from 
one of the tests, was ‘5 x 68°9, ‘5 being the rise in temperature, and 68°9 the capacity 
_ of the calorimeter. The maximum percentage error due to radiation in a length of 
4 centims. is therefore 5°5 per cent. This is on the assumption that there is no 
jacket round the wire, but even then the actual loss would probably be less than 
3 per cent. if currents of air be avoided. This rough calculation shows that radiation 
from the surface of the wire need not be considered as an objection to the method. 
2, Possible error due to the fact that the thermometers are not actually at the 
ends of the wire, and so may not be indicating the proper temperatures. 
The effect of this error would also be to give too low a value for the conductivity. 
To guard against this the bottom of the box is made very thick, and the large block 
K (fig. 2) is added so as to hold the thermometer. The difference of temperature 
between the inside and outside surfaces of the bottom of the box is certainly 
exceedingly small, if as much heat as the wire can take away is supplied to it. 
Since the heat is supplied by boiling water, it is, however, possible that the copper 
conducts so quickly that there is always a layer of water, it may be thin, immediately 
in contact with the metal at a very much lower temperature than 100° C. This 
point is particularly emphasized by Lord KELvIn in volume 8 of his Collected Papers, 
where he remarks upon the exceedingly low values obtained by CLEMENT and by 
Pécier for the conductivity of copper. These experimenters both attempted to 
measure the conductivity by keeping one side of a slab of the metal in contact with 
water at a constant temperature and measuring the rise in temperature of a known 
MDCCCXCY.—A. Z 
