IX. The thermal emissivity of thin wires in air

Abstract

In 1884 it was observed experimentally that whereas the electric current required to maintain a thick wire of given material, under given conditions, at a given temperature, was roughly proportional to the diameter of the wire raised to the power three-halves, the current was more nearly proportional to the first power of the diameter if the wire were thin . When this difference in the behaviour of a thick and a thin wire was first noticed it was regarded as quite unexpected. But, as pointed out by one of us in the course of a discussion at a meeting of the Royal Society, the unexpected character of the result was due to people having assumed that the loss of heat from radiation and convection per square centimetre of surface per 1° excess temperature was a constant for a given kind of surface and independent of the size and shape of the cooling body, although as early as 1868 Box had drawn attention to the great difference that existed between the rate of loss of heat from unit area of a horizontal cylinder and per unit area of a sphere. The interchange of heat between unit area of a body and the enclosure might be independent of the shape of the body as far as radiation alone was concerned, but it seemed nearly obvious that the cooling by convection must be materially affected by the shape of the cooling body. The very valuable investigations that have been made on emissivity by Mr. Macfarlane, Professor Tait, Mr. Crookes, Mr. J. T. Bottomley, and by Mr. Schleiermacher, had for their object the determination of the variation of the emissivity with changes of the surface and with change in the density of the gas surrounding the cooling body, but it was not part of these investigations to determine the change in the emissivity that is produced by change in the shape and size of the cooling body. Indeed, so little has been the attention devoted to the very large change that can be brought about in the value of the emissivity by simply changing the dimensions of the cooling body, that in Professor Everett’s very valuable book on Units and Physical Constants, the absolute results obtained by Mr. Macfarlane are given as the “results of experiments on the loss of heat from blackened and polished copper in air at atmosphere pressure,” and no reference is made either to the shape or to the size of the cooling body.