276 



NEGEETTI AND ZAMBEA, HOLBOEN VIADUCT, E.G., 



CEOOKES' EADIOMETEE. 



This Instrument demonstrates the supposed conversion 

 of Light into Mechanical Motion, invented by Mr. William 

 Crookes, F.B.S., and first exhibited by him at the Soiree 

 of the Royal Society, April 7th, 1875, and described by 

 him in the Quarterly Journal of Science, for July, 1875, as 

 follows : 



" The Instrument which I have called a Radiometer, 

 shown in fig. 1049, consists of four arms, of some light 

 material, suspended on a hard steel point resting in a 

 cup, so that the arms are able to revolve horizontally 

 upon the centre pivot, in the same manner as the arms of 

 Dr. Robinson's Anemometer revolve. To the extremity of 

 each arm is fastened a thin disc of roasted mica or pith, 

 white on one side and lamp-blacked on the other, the 

 black surfaces of all the discs facing the same way. The 

 whole is enclosed in a thin glass globe, which is then 

 exhausted to the highest attainable point and hermetically 

 sealed. 



" The arms of this Instrument rotate with more or less 

 velocity under the action of radiation, the rapidity of 

 revolution being directly proportional to the intensity of 

 the incident rays. Placed in the Sun, or exposed to the 

 light of burning Magnesium, the rapidity is so great that 

 the separate discs are lost in a circle of light. Exposed 

 FIG 1049 ^0 a Candle 20 inches off another instrument gave one 



revolution in 182 seconds. With the same Candle placed at 



a distance of 10 inches off the result is one revolution in 45 seconds ; and at 5 inches 

 off one revolution was given in 11 seconds. Thus it is seen that the mechanical 

 action of radiation is inversely proportional to the square of the distance. At the 

 same distance 2 Candles give exactly double, and 3 Candles give three times, the 

 velocity given by 1 Candle, and so on up to 24 Candles. A small Radiometer was 

 found to revolve at the velocities shown in the following table, when exposed to 

 the radiation of a standard Candle 5 inches off. 



Time Required for One Revolution. 



Source of Radiation. Time in Seconds. 



1 candle, 5 inches off. behind green glass 40 



5 blue 38 



5 purple 28 



5 orange ., 26 



5 yellow 21 



5 light red,, 20 



" The position of the light in the horizontal plane of the Instrument is of no 

 consequence, provided the distance is not altered ; thus' two Candles, 1 foot off, give 

 the same number of revolutions per second, whether they are side by side or 

 opposite to each other. From this it follows that if the radiometer is brought 

 into a uniformly lighted space it will continue to revolve. 



" In diffused daylight, the velocity was one revolution in from 1*7 seconds to 2'3 

 seconds, according to the'intensity of the incident rays. In full Sunshine, at 10 A.M., 

 it revolved once in 0'3 second, and at 2 P.M. once in 0'25 second. 



" When heat is cut off by allowing the radiation to pass through a thick plate 

 of Alum, the velocity of rotation is somewhat slower." 

 1049 Crookes' Radiometer, with Black and White Discs (as fig. 1049) on Stand 



10 6 



Ditto ditto, with Double Vanes Rotating in opposite directions, on 

 Stand . 110 



For further details of Preliminary Experiments and Researches, &c., &c., 

 see Pamphlet, Crookes' Radiometer. Price Is. 



