Abstract
The experiments here described have arisen from preliminary work conducted with the ultimate object of testing the late Prof. Callendar's formula* for the distribution of energy in the spectrum. The latter experiments, not yet completed, involved radiation-pressure determinations made with a vane suspended in a vacuum sufficiently high for radiometer action to be completely eliminated. While the manner in which radiometer action diminishes as the vacuum improves was being investigated, the series of radiometer effects recorded in the first part of this paper was observed. Radiation was directed upon a light vane suspended by means of a fine quartz fibre in a flask, and the deflections produced when the radiation was incident in turn upon the two sides of the vane were observed for a series of vacuum conditions. The vanes employed included platinized or silvered glass and mica, and also an aluminium vane. The range of air pressure used extended down to about 10-6 mm. of mercury.
In the case of the platinized glass vane, at a pressure of a few mm. of mercury the deflections on both the glass and the platinized sides were of the order of those registered at the highest vacua obtained. At pressures of about 10-2 mm. gas effects developed which were very large in the case of the glass and mica vanes but much less marked with the aluminium vane. With all the vanes, gas action had practically vanished at a pressure of 10-6 mm. of mercury. Under these conditions, measurements of radiation pressure, using glass and mica vanes, were found to give results in agreement with the energy-density of the radiation to within ± 7 per cent.
While the above preliminary experiments were in progress the errors of calculation occurring in Nichols and Hull's paper dagger on radiation pressure were noticed. Although their work, carried out at a gas pressure of 16 mm. of mercury, has been widely quoted as conclusively establishing, to within about 1 per cent, the numerical equivalence between the pressure and energy-density of radiation, it is found that their results, when correctly evaluated, show a divergence between these quantities of some 10 per cent. Hence Nichols and Hull's investigation cannot be regarded as furnishing a quantitative experimental verification of the equality relationship deducible from theory.
It was therefore thought worth while, using the working experience gained, to attempt more accurate measurements of radiation pressure by the direct method of using metal vanes and a vacuum so high that radiometer action was eliminated. In the second part of the paper the observations and results for all the radiation-pressure measurements are given. With each vane a considerable range of radiation-intensities was employed, the maximum deflections of the suspensions under the influence of the radiation-pressure being up to 10 or 15 times those obtained by Nichols and Hull. The corresponding energy-densities of the radiation were measured by the use of a Callendar cup radio-balance. The results for an alloy vane, probably the most reliable of those derived, show that for a number of radiation-intensities the difference between pressure and energy-density varies from +4 to -3 per cent. The present experiments are considered in the light of those conducted by other observers. Errors of calculation arising in Nichols and Hull's results are examined in the appendix.