THERMAL EMISSIYITY OF THIN WIRES IN AIR. 
373 
ernissivity, it is also convenient to have some concrete conception of the wires spoken 
of. Now, fine wires are practically known in this country as wires of 1, 1|, 2 mils, 
&c., diameter, a wire of 1 mil diameter being, for example, the thinnest that has been 
practically used in the construction of electrical apparatus. A better idea is, there¬ 
fore, obtained from stating that the diameters of wires are 1, 2, or 3 mils, than from 
saying that they are 0'025, 0 - 051, or 0'074 millim. Attached to several of the 
curves are the diameters of the wires expressed in millimetres. These numbers are 
stated to four decimal places, but it would have been better to have given, as in the 
above Table, only three significant figures, this being the probable limit to the 
accuracy of the measurement of the diameters. 
Suspecting that some of the results of published experiments on the currents 
required to fuse wires had been much influenced by the cooling action of the blocks 
to which the ends of the wires were attached, we started by making a calculation on 
the length necessary to give to our wires so that the loss of heat by conduction 
should not introduce any important error into the determination of the ernissivity. 
To do this it was necessary to calculate the distribution of temperature along a wire 
through which a steady current was flowing, and from which heat was lost by 
radiation, convection, and conduction, and it was further necessary to improve on the 
calculation one of us had published on this subject in the ‘ Electrician,’ for 1879, by 
now taking into account the fact that the ernissivity, as well as the thermal and 
electric conducting powers of the wire, were different at different points in conse¬ 
quence of the difference of temperature. Such a calculation has not, as far as we are 
aware, been hitherto made, it having been assumed in all previous investigations that 
the effect due to the variation of the thermal and electric conducting power of the 
material with temperature, as well as the variation of the ernissivity per square 
centimetre with temperature and with the diameter of the wire, could be neglected. 
Until we had completed the experiments described in this paper we could of course 
only employ, in this calculation, values that we had guessed at as something near the 
truth for the ernissivity of platinum wire for different diameters and at different 
temperatures. Hence, after the completion of the experiments, we took up the 
mathematical investigation again, substituting for the ernissivity such a function of 
the diameter of the wire and the temperature of the point as we had experimentally 
found it to be. The investigation by which we finally arrived at the calculated 
distribution of temperature along the wire is given in § Y. of the paper. 
The rate at which heat was lost by any one of the wires was measured by the 
product of the current passing through it into the P. D. (potential difference) main¬ 
tained between its ends, while the ratio of the P. D. to the current gave the resist¬ 
ance of the wire, and therefore its temperature. As the variation of resistance with 
temperature of different specimens of platinum is knowm to differ, it was not con¬ 
sidered sufficiently accurate to deduce the temperature of the wire experimented on 
by using some supposed temperature coefficient for platinum ; consequently the 
variation of resistance, with temperature of each piece of platinum wire employed, 
