THE RADIOMETER NOT A LIGHT MOTOR.

Thames Star, Volume VII, Issue 2615, 26 May 1877, Page 3

THE RADIOMETER NOT A LIGHT MOTOR.

(from thh scientific American.) The imnrerise quantity of evidence in the shape of experiments on the radiometer, which has been accumulated, by the principal physicists of Europe, leaves .00 reasonable doubt but that the claim that the apparatus is., driten by light is unfounded. It is maintained by our' correspondent, M. Delsaulr, that the radiometer is an; electrical engine, a majority of observers pronounce it simply a machine driven by heat. So long as it is decided that the radiometer is actuated by forces which are iutrinsically perfectly well understood, leaving the problem merely one of determining how the - same are applied, the whole subject sinks at once to secondary importance. - It was only because the apparatus appeared to demonstrate the ponderability of light, and hence was an apparent serious con' 1 tradiction'to the undulatory theory, that it has excited the close attention of the scientific world. Dr Frankland's recent experiments on the radiometer are among the most conclusive that have been made, as showing the true source of its motion. The discs of his instrument were of aluminium, polished on one' side and blackened on the other.' - They were • extraordinarily sensitive, and continued rotating often for 20 minutes after the sun had descended below the horizon. Placing this radiometer in a room where the conditions were such that it remained motionless Dr Frankland, enclosed its globe in his hands so as cut off .light from the discs. Y"et they at once began to rotate, polished side, in advance. On withdrawing his lands, the motor, after the lapse of two or'three- minutes, stopped. Then, after another brief interval, it resumed in the opposite -direction for a short time. There was rotation set up without the aid-of light, and clearly due to the temperature of the investigator's hands. The'next experiment was exactly the converse,, and involved testing the apparatus under light destitute of heat. It is well known that the moon although reflecting the sun's rays, sends but a rery small amount of heat to the earth, With a lens 3ft in diameter, concentrating the lunar rays on a.thermoscopic pile, Melloni proved that the deviation of the needle was from Odeg. 6min. to 4d°ff--Bmm., according to the phase of the moon. This indicates so extremely small a heating power that it is.- practically inconsiderable, and. is destitute of any. known effects upon the earth. Dr Frankland, therefore, exposed his radiometer to bright moonlight, but it did not stir. Then, with a powerful lens, he rendered the intensity of the rays 200 times greater, and directed the brilliant image of the moon, produced at the focus, directly upon one of the discs of the radiometer. Wot the slightest movement was appreciable, although the light was kept unaltered for over a quarter «f an hour. Dr. Frankland's conclusions sum up the true facts regarding the radiometer in brief terms.- Ho says that light is not necessary.to the motion of the instrument, that light does not contribute to its motion unless (by absorption) it be transformed into heat, which did not take place (or occurred in an unappreciable degree) for moonlight, that the movement of the discs is due to the unequal heating of the two faces of each disc, the cooler face always preceding the hotter one. "When Jl^.globe is taken in the hands, the blackened face of. each disc absorbs heat rapidly, while the bright side reflects it. Thus the' surfaces of the black > discs remain hotter than the motal situated below, but soon communicate their heat to said metal. -If tho.hands be withdrawn, the thermal conditions are reversed;, the -black face, being a better absorbent and at the same titae a better radiator, cools much more quickly, and hence the opposite motion of the mill already described.