MENT. (S. RADIOMETER, an instrument for measuring the intensity of the radiant energy of rarefied gases. It was noticed by Fresnel that a body delicately suspended in vacuo is apparently repelled by radiation. Sir W. Crookes was the first to make systematic experiments; he found that a light vane blackened on one side and bright on the other was repelled if radiation was allowed to fall on the blackened face. He constructed an instrument, which he called a radiometer or light-mill, by pivoting a vertical axle carrying vertical vanes inside an exhausted bulb ; one side of each vane was blackened and the other side bright, the black sides all facing the same way round the axle. When rays from the sun or other light source, or dark radiation from a warm body, fall on the vane, the black sides are repelled more than the bright sides, and the vanes are set into rotation. The rate of rotation rises to a value which depends on the intensity of the radiation; the more intense the radiation the more rapidly do the vanes rotate.
That this is so is important, for it indicates that the force opposing the motion is one which increases as the rate of rotation increases. Now the friction of the pivot is independent of the rate of rotation, so that the main opposing force must be that due to the residual gas in the bulb, for the viscous drag on a body in motion through a gas increases as the speed increases.
The name "radiometer" arose from Crookes's idea that the instrument might be used to measure the intensity of radiation; with the instrument he constructed he was able to detect the radiation from a candle 19 meters away.
That it was necessary to blacken one side of each vane sug gested that the effect was due to a difference of temperature between sides of the vane caused by the greater absorbing power of the black coating, and the question arose whether the rotation was caused by the direct impact of the radiation (somewhat as the rotation of a cup anemometer is caused by the wind), or whether it was caused by difference of pressure of the gas set up by the inequalities in temperature. This was settled by a very
beautiful experiment performed by Sir A. Schuster. He suspended the case of the radiometer by a fine thread so that it was free to rotate about a vertical axis; a small mirror fixed to the case, which reflected a beam of light on to a scale, enabled him to ob serve any rotation. If the vanes were driven round by the direct action of radiation, then the viscous drag of the gas acting also on the inside of the bulb would drag the latter round in the same sense as the vane. If, on the other hand, the rotation were caused by stress in the gas, the couple which set the vanes in rotation would involve an equal and opposite couple on the case, and the case would thus begin to rotate in the opposite sense. As the rate of rotation of the vanes increased, the viscous opposition of the gas would increase until, when the forces due to viscosity be came equal to the driving forces, the resultant on both the vane system and on the case would vanish ; the vanes would continue in uniform rotation and the case return to its position of equilibrium. When the rotation ceased to fall on the vanes, there would no longer be any driving couple and the viscosity of the gas would bring the vanes to rest and impel the case in the direction of their rotation.
The experiment completely confirmed this second hypothesis, namely that the rotation of the vanes is due to mutual forces be tween them and the case. This was further confirmed by sus pending the case in an inverted position so that the vane system was no longer free to rotate unless it took the case round with it. There was then no motion of the case. From Schuster's measure ments of the angular deflection of the light-mill supported by a hi-filar suspension, 0. Reynolds calculated the magnitude of the pressure difference on the two sides of the vane to be of the order of one thousandth of the gas pressure in the bulb.