THERMAL EMISSIVITY OF THIN WIRES IN AIR. 
III. Results of Emissivity Experiments. 
To determine the temperature of any one of the wires used in the emissivity 
experiments from its observed resistance, we might proceed as follows : calculate the 
specific resistance and then determine from the preceding curves, 1, 2, 3, &c. (figs- 7 
and 8), the temperature at which a piece of the same wire had the same specific 
resistance. To do this it would be, of course, necessary to know the lengths of the 
two pieces of the same wire used in the two sets of experiments. 
Now, whereas in each of the emissivity experiments a straight piece of wire of 
about 28 centims. was employed, the length of which could be measured with con¬ 
siderable accuracy, it was only possible to use shorter pieces for the experiments on 
the variation of resistance with temperature, for the latter pieces were parts of the 
wires that had actually been used in the emissivity experiments, and detaching 
a piece of wire from the apparatus seen in figs. 1 to 4, and attaching it to the clamp 
shown in figs. 5 and 6, necessarily shortened its length. 
Since, therefore, there was this greater difficulty in measuring the exact lengths of 
the wires used in the experiments recorded in Tables I, to V., curves 1, 2, 3, &c., figs. 
7 and 8, it was decided to regard these experiments as giving the relative resistances 
of each particular wire at different temperatures, but not the specific resistance at any 
particular temperature. 
In order then to use these curves of relative resistance with temperature 1, 2, 3, &e., 
for the purpose of determining the temperature of the wires in the emissivity experi¬ 
ments, we must know the resistance of each of the latter wires at some one tempera¬ 
ture. This might have been ascertained by means of the Wheatstone’s bridge, but, 
as a relative resistance rather than absolute resistance was required, the following 
method was employed instead, since it avoided the possible introduction of an error 
that might have arisen from some want of agreement in the unit of resistance of the 
Wheatstone’s bridge and the units of current and P. D. The ratio of P. D. to current, 
or the resistance of the wire, having been measured for a number of currents, a curve 
was plotted connecting the resistance of the wire for various currents flowing through 
it, and by continuing this curve until it cut the axis along which resistance was 
measured we obtained the limiting value of the ratio P. D. to current for current 
nought, that is, the resistance of the wire when at the temperature of the enclosure. 
Pig. 9 (Plate 13) shows the curve thus obtained for the thickest wire, viz., that of 
14 mils, and similar curves were drawn for all the other wires for the purpose of 
ascertaining their respective resistances when at the temperature of the enclosure. 
Let / and d be the length and diameter of the wire in centimetres, t 0 be the tempera¬ 
tures ot the water jacket surrounding the enclosure cylinder ABCD (figs. 1 and 2); 
t be the temperature of the wire when A amperes are passing through it, and when 
V volts is the P. D. between the ends of the wire, e be the emissivity, that is the 
MDCCCXCII.—A. 3 D 
