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RADIOMETER
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RADIOMETER . It had been remarked at various times, amongst others by See also:Fresnel, that bodies delicately suspended within a partial vacuum are subject to apparent repulsion by See also:radiation. The question was definitely investigated by See also:Sir W. See also:Crookes, who had found that some delicate weighings in vacuo were vitiated by this cause. It appeared that a See also:surface blackened so as to absorb the radiant See also:energy directed on it was repelled relatively to a polished surface. He constructed an apparatus in See also:illustration, which he called a radiometer or See also:light-See also: On the otherhand, if the effects arose from balanced stresses set up inside the globe by the radiation, the effects on the vanes and on the case would be of the nature of See also:action and reaction, so that the See also:establishment of motion of the vanes in one direction would involve impulsion of the case in the opposite direction; but when the motion became steady there would no longer be any See also:torque either on the vanes or on the case, and the latter would therefore come back to its previous position of equilibrium; finally, when the light was turned off, the decay of the motion of the'vanes would involve impulsion of the case in the direction of their motion until the moment of the restoring torque arising from the suspension of the case had absorbed the angular momentum in the See also:system. Experiment showed that the latter prediction was what happened. The important See also:part played by the residual air in the globe had also been deduced by See also:Osborne See also:Reynolds from observing that on turning off the light, the vanes came to See also:rest very much sooner than the friction of the pivot alone would See also:account for; in fact, the rapid subsidence is an illustration of See also:Maxwell's See also:great theoretical See also:discovery that viscosity in a See also:gas (as also See also:diffusion both of See also:heat and of the gas itself) is sensibly See also:independent of the See also:density. Some phenomena of retardation in the See also:production of the effect had led Sir G. G. See also:Stokes and Sir W. Crookes to the same See also:general conclusion. The origin of these phenomena was recognized, among the first by O. Reynolds, and by P. G. See also:Tait and J. See also:Dewar, as a consequence of the kinetic theory of the constitution of gaseous See also:media. The temperature of a gas is measured by the mean energy of See also:translation of its molecules, which are independent of each other except during the brief intervals of collision; and collision of the See also:separate molecules with the blackened surface of a vane, warmed by the radiation, imparts heat to them, so that they rebound from it with greater velocity than they approached. This increase of velocity implies an increase of the reaction on the surface, the See also:black side of a vane being thus pressed with greater force than the bright side. In air of considerable density the mean See also:free path of a See also:molecule, between its collisions with other molecules, is exceedingly small, and any such increase of gaseous pressure in front of the black surface would be immediately neutralized by flow of the gas from places of high to places of See also:low pressure. But at high exhaustions the free path becomes comparable with the dimensions of the glass bulb, and this equalization proceeds slowly. The general nature of the phenomena is thus easily understood; but it is at a maximum at pressures comparable with a milli-See also:metre of See also:mercury, at which the free path is still small, the greater number of molecules operating in intensifying the result. The problem of the stresses in rarefied gaseous media arising from inequalities of temperature, which is thereby opened out, involves some of the most delicate considerations in molecular physics. It remains practically as it was See also:left in 1879 by two See also:memoirs communicated to the Phil. Trans. by Osborne Reynolds and by Clerk Maxwell. The method of the latter investigator was purely a priori. He assumed that the See also:distribution of molecules and of their velocities, at each point, was slightly modified, from the exponential See also:law belonging to a uniform See also:condition, by the gradient of temperature in the gas (see DIFFUSION). The See also:hypothesis that the See also:state was steady, so that interchanges arising from convection and collisions of the molecules produced no aggregate result, .enabled him to interpret the new constants involved in this law of distribution, in terms of the temperature and its spacial See also:differential coefficients, and thence to See also:express the components of the kinetic stress at each point in the See also:medium in terms of these quantities. As far as the See also:order to which he carried the approximations—which, however, were based on a simplifying hypothesis that the molecules influenced each other through mutual repulsions inversely as the fifth See also:power of their distance apart--the result was that the equations of motion of the gas, considered as subject to viscous and thermal stresses, could be satisfied by a state of equilibrium under a modified internal pressure equal in all directions. If, therefore, the walls of the enclosure held the gas that is directly in contact with them, this equilibrium would be the actual state of affairs; and it would follow from the principle of See also:Archimedes that, when extraneous forces such as gravity are not considered, the gas would exert no resultant force on any body immersed in it. On this ground Maxwell inferred that the forces acting in the radiometer are connected with gliding of the gas along the unequally heated boundaries; and as the See also:laws of this slipping, as well as the constitution of the adjacent layer, are uncertain, the problem becomes very intricate. Such slipping had shown itself at high exhaustions in the experiments of A. A. See also:Kundt and E. G. Warburg in 1875 on the viscosity of gases; its effects would be corrected for, in general, by a slight effective addition to the thickness of the gaseous layer. Reynolds, in his investigation, introducing no new See also:form of law of distribution of velocities, uses a linear quantity, proportional to the mean free path of the gaseous molecules, which he takes to represent (somewhat roughly) the See also:average distance from which molecules directly affect, by their convection, the state of the medium; the gas not being uniform on account of the gradient of temperature, the See also:change going on at each point is calculated from the elements contributed by the parts at this particular distance in all directions. He See also:lays stress on the dimensional relations of the problem, pointing out that the phenomena which occur with large vanes in highly rarefied gas could also occur with proportionally smaller vanes in gas at higher pressure. The results coincide with Maxwell's so far as above stated, though the numerical coefficients do not agree. According to Maxwell, priority in showing the See also:necessity for slipping over the boundary rests with Reynolds, who also discovered the cognate fact of thermal transpiration, meaning thereby that gas travels up the gradient of temperature in a capillary See also:tube, owing to surface-actions, until it establishes such a gradient of pressure (extremely See also:minute) as will prevent further flow. In later memoirs Reynolds followed up this subject by proceeding to establish See also:definitions of the velocity and the momentum and the energy at an See also:element of See also:volume of the molecular medium, with the precision necessary in order that the dynamical equations of the medium in bulk, based in the usual manner on these quantities alone, without directly considering thermal stresses, shall be strictly valid—a discussion in which the relation of See also:ordinary molar See also:mechanics to the more See also:complete molecular theory is involved. Of See also:late years the peculiarities of the radiometer at higher gas-pressures have been very completely studied by E. F. See also:Nichols and G. F. See also:Hull, with the result that there is a certain pressure at which the molecular effect of the gas on a pair of nearly vertical vanes is balanced by that of convection currents in it. By thus controlling and partially eliminating the aggregate gas-effect, they succeeded in making a small radiometer, horizontally suspended, into a delicate and reliable measurer of the intensity of the radiation incident on it. With the experience thus gained in manipulating the vacuum, the achievement of thoroughly verifying the pressure of radiation on both opaque and transparent bodies, in accordance with Clerk Maxwell's See also:formula, has been effected (See also:Physical See also:Review, 1901, and later papers) by E. F. Nichols and G. F. Hull; some months earlier Lebedew had published in the Annalen der Physik a verification for metallic vanes so thin as to avoid the gas-action, by preventing the production of sensible difference of temperature between the two faces by the incident radiation. (See RADIATION.) More recently J. H. Poynting has separated the two effects experimentally on the principle that the radiometer pressure acts along the normal, while the radiation pressure acts along the See also:ray which maybe directed obliquely. (J. L. Additional information and CommentsThere are no comments yet for this article.
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