180 MR. J. H. GRAY ON A METHOD OF DETERMINING 
their mean, we get for the thermal conductivity of the laboratory ccpper wire 
8884 O.G.S. unit. This value is the mean conductivity between the temperatures 
97- C. and 10° €. 
The following Table gives the resuits of a series of tests on another portion of the 

same wire :— 
Length in centims. Conductivity. 
6:11 888 
571 “883 
4°80 “892 
4°43 *889 





The mean of these values is °8880, being 3'; per cent. less than that obtained from 
the preceding series. 
Taking Anastrom’s formula, 
K = 1°027 (1 — :00214¢), 
and finding K for 53°, which is the mean of 97° and 10°, we get K = ‘9208 C.GS. 
unit. 
The close agreement in the values obtained with different lengths shows that the 
errors already mentioned as possible are practically eliminated. ANGsTROm’s value is 
4°5 per cent. greater thau *8884, but, as has been mentioned (p. 176), the conductivity 
of the laboratory copper wire is 8 per cent. less than that of GLoveER’s wire afterwards 
tested. In a previous paper, ANGsTROM gives ‘91 as the value for 513° C. 
In a paper read before the Royal Society last year, Dr. R. W. Stewart, using the 
Forbes method, but substituting a single thermo-electric junction for the thermometers, 
gives the values for copper and iron, At the temperature 53° C. he gets for copper 
K = 1-067, which is 10 per cent. higher than the value obtained for any specimen of 
copper tested in the present investigation. 
The conductivity of the wire obtained from Messrs. GLovER and Son was found to 
be 9594 C.G.8. This was the best conducting copper tested by the present method. 
The specific electrical resistance was 1730 in absolute units. 
To test separately the effect of any alterations, separate experiments were made. 
I. Instead of enclosing the whole length of the wire (laboratory copper) in a tube, 
only a part was enclosed. 
The paper tube was shortened by 2 or 3 millims. each time, and the conductivity 
of the same length of wire was determined after each shortening of the tube. The 
result found was, that the shortening (which was, of course, done from the cool end of 
the wire) had practically no effect on the value so long as it was arranged that there 
were no draughts of cold air. If for no other reason, the paper tube is necessary to 
