CHAPTER V
FULL RADIATION
The Full Radiator.—We have assumed that a lamp-blacked surface is a perfect absorber, and consequently a full radiator, but although it is a very near approach to the ideal it is not absolutely perfect. No actual surface is a perfectly full radiator, but the exact equivalent of one has been obtained by an ingenious device. A hollow vessel which is blackened on the inside has a small aperture through which the radiation from the interior of the vessel can escape. If the vessel is heated up, therefore, the small aperture may act as a radiator. The radiation which emerges through the aperture from any small area on the interior of the vessel is made up of two parts, one part which it radiates itself, and the other part which it scatters back from the radiation which it receives from the other parts of the interior of the vessel. These two together are equal to the energy sent out by a full radiator, and therefore the small aperture acts as a full radiator: e.g. suppose the inner surface has an absorbing power of 90 per cent., then it radiates 90 per cent. of the full radiation and absorbs 90 per cent. of the radiation coming up to it therefore scattering back 10 per cent. We have therefore coming from the inner surface 90 per cent. radiated and 10 per cent. scattered, and the radiated and scattered together make 100 per cent.
FIG. 22.
One form in which such radiators have been used is shown in section in Fig. 22. A double walled cylindrical vessel of brass has a small hole, a, in one end. Steam can be passed through the space between the double walls, thus keeping the temperature of the inner surface at 100° C. A screen with a hole in it just opposite to the hole in the vessel, or rather several such screens, are placed in front of the vessel in order to shield any measuring instrument from any radiation except that emerging through the hole.
The Full Absorber.—In an exactly similar way an aperture in a hollow vessel will act as a full absorber, for the fraction of the incident radiation which is scattered on the inner surface again impinges on another portion of the surface and so all is ultimately absorbed except a minute fraction which is scattered out again through the aperture.
The variation in the heat radiated by a full radiator at different temperatures forms a very important part of the study of radiation, and a very large number of experiments and theoretical investigations have been devoted to it. These investigations may be divided into two sections: those concerned with the total quantity of heat radiated at different temperatures and those concerned with the variation in the character of the spectrum with varying temperatures.
The experiments in the first section have been carried out mainly in two ways. In the first, the rate of cooling of the full radiator has been determined, and from the rate of cooling at any temperature the rate at which heat was lost by radiation was immediately calculated. Newton was the first to investigate in this way by observing the rate at which a thermometer bulb cooled down when it was surrounded by an enclosure which was kept at a uniform temperature. He found that the rate of cooling, and therefore the rate at which heat was lost by the thermometer, was proportional to the difference of temperature between the thermometer and its surroundings. This rule is known as Newton's Law of Cooling, and is still used when it is desired to correct for the heat lost during an experiment where the temperature differences are small. It is only true, however, for very small differences of temperature between the thermometer and its surroundings, and as early as 1740 Martine had found that it was only true for a very limited range of temperature.