206 
Mil. H. L. CALLENDAR ON THE PRACTICAL 
In this experiment the wire is radiating freely, so that it attains, not the tempera¬ 
ture of the flame, but a point at which the loss of heat by radiation balances the heat 
received from the impact of the burning gas. The more rapid the current, the higher 
the temperature reached ; this accounts partly for the higher temperature to which 
the spiral was raised in the edge of the flame, where the current is much stronger as 
well as the supply of air better. By suitably screening the wire it would probably be 
possible to raise it to a higher temperature. The temperature can only be roughly 
estimated by the radiation, because the latter is selective. If the wire be blackened 
(platinised) it appears much brighter, because it radiates better, but it is for that very 
reason not so hot, as the measurement of its resistance shows. 
Appendix. 
Received June 9, 1887. 
Since the foregoing paper was communicated to the Royal Society, so many more 
extended and accurate experiments have been made that it was thought desirable to 
delay the publication of the paper for a few months, in order that some account of 
them might be included. It was felt that this would render the paper more useful 
without adding excessively to its length. The observations witli the air thermometer 
in particular have been rendered more accurate, so that the temperature may be 
determined by aid of the platinum wire with a probable error of less than a degree at 
600° C. A much improved method of representing the results graphically has also 
been adopted (see Plate 13), which shows at a glance the kind of accuracy attained. 
More accurate and extended comparisons tend to confirm the general conclusions from 
the earlier experiments : that the curves of resistance-variations of different wires of 
the same metal are similar, but not coincident; that the usual parabolic formulfe give 
an accuracy, of the order of 1 per cent, only, through a range 0°-600° C. ; that the 
deviations therefrom are not readily expressible by any simple formulae. 
Considerable improvements have been made in the primitive apparatus. An 
improved form of gauge was devised and ordered about the middle of June 1886, 
which was subsequently found to work well and to possess many advantages over 
the primitive kind; the principle was exactly the same as that described on page 172, 
but the mechanical provisions were more complete and perfect. They will be readily 
understood on reference to the diagram, fig. 6, Plate 12. The sulphuric acidU-gauge 
proper is represented by the tube FGHJK; it has a three-way tap T 1; which may 
be turned in the middle of the acid, so that it cannot move either way, whatever the 
variations of pressure : this is a most important practical improvement. 
The requisite amount of acid is introduced through the side-tube P l9 and the scale 
