THE RADIOMETER AND ITS LESSONS. 



27 



a bar of pith by a fibre of cocoon-silk within a 

 glass globe, so as to constitute what is known as 

 a " balance of torsion," and subjecting one end 

 of this bar to the influence of heat. When the 

 globe is full of air, the warmed end of the bar 

 swings toward the source of heat ; but, when the 

 globe has been thoroughly exhausted and her- 

 metically sealed, the bar is made to swing away 

 to the extent of 90°, by merely touching with the 

 finger the part of the globe near one of its ex- 

 tremities ; while, on the other hand, it follows a 

 piece of ice as a suspended needle follows a 

 magnet. These contrary effects are very strik- 

 ingly shown when two similar globes, each hav- 

 ing a pith-bar suspended in it, but the one full ot 

 air and the other exhausted, are placed side by 

 side, and a hot glass rod on the one hand, and a 

 piece of ice on the other, are moved round each 

 in succession. For the bar in the unexhausted 

 globe behaves exactly with the heated rod as the 

 bar in the exhausted globe does with the ice ; 

 and the bar in the unexhausted globe behaves 

 with the ice exactly as the bar in the exhausted 

 globe behaves with the heated rod. Again, when 

 a candle is brought within about two inches of 

 a well-exhausted globe, the pith-bar begins to 

 oscillate backward and forward, its swing grad- 

 ually increasing until its position is reversed ; and 

 when the "dead centre" has been passed it re- 

 volves continuously, until the torsion of the sus- 

 pended fibre offers a sufficient resistance to prevent 

 any further movement in the same direction. A 

 contrary revolution then begins, which proceeds 

 as far in the opposite direction, the alternating 

 series of revolutions being kept up as long as the 

 candle burns. 



A still more sensitive apparatus of the like 

 kind was afterward devised by Mr. Crookes, in 

 which two disks of pith were attached to the 

 extremities of a very slender glass rod, and this 

 was suspended horizontally by a fine fibre of 

 spun glass; the whole being hermetically sealed 

 within a glass vessel of suitable form, from 

 which the air was removed as completely as 

 possible. The advantage of suspending the beam 

 by a glass fibre lies in its elasticity; which, is so 

 perfect, that, however much the fibre may have 

 been twisted, the beam always returns accurately 

 to zero when free to do so. And by drawing out 

 the fibre to the requisite degree of fineness, this 

 " torsion-balance " may be made of any degree of 

 sensitiveness that may be required ; one which was 

 used in Mr. Crookes's subsequent experiments be- 

 ing so delicate as to turn to the millionth of a grain. 



From this form of apparatus, the transition 



was obvious to one in which the arms, instead 

 of being suspended by a fibre, should rest on a 

 point, so as to be free to rotate continuously in 

 either direction ; and thus originated the radiom- 

 eter — the name given to it by Mr. Crookes being 

 intended to express its action as a measurer of 

 the mechanical power directly exerted by that 

 radiant energy which had been previously known 

 to manifest itself only under the forms of light, 

 heat, and actinism (or chemical agency). This 

 was the sense in which its phenomena were 

 brought under discussion at the ordinary meet- 

 ing of the Royal Society next following the first 

 exhibition of the radiometer; and so demon- 

 strative did these seem to be of " A Eepulsion 

 resulting from Radiation " (the title of Mr. 

 Crookes's memoir), that no one of the eminent 

 physicists present on the occasion called his in- 

 terpretation of them in question ; Prof. Stokes, 

 in particular, confining himself to the statement 

 that such mechanical action must lie outside the 

 undulatory theory, which deals only with light 

 as light — i. e., as producing visual phenomena. 

 But it was noticed by several as anomalous, that 

 the black should be the "driving" side of the 

 disks, since it might have been anticipated that 

 the mechanical action of light would manifest 

 itself in pushing away the surface from which its 

 rays are reflected, and that the surface into which 

 they are absorbed would move toward the source 

 from which the rays emanate. 



In subsequent communications to the Royal 

 Society, Mr. Crookes committed himself explicitly 

 to the doctrine that the radiometer (to which he 

 also gave the name of " light-mill ") is driven by 

 light, the mechanical effect of which he assumed 

 to be proportional to its illuminating power ; so 

 that an exact measurement of the former would 

 furnish an equally exact measurement of the lat- 

 ter. And thus, as a thermometer, which meas- 

 ures heat by its physical action in producing ex- 

 pansion, is a far more trustworthy instrument 

 than the human hand, the action of heat upon 

 which gives rise to a sensation that is not capable 

 of precise measurement, and may be altogether 

 deceptive, so he contended the radiometer, which 

 measures light by its physical power of repulsion, 

 is a far more exact photometer than any which 

 depends upon the physiological action of light 

 upon the retina. 



For the absolute measurement of the repulsive 

 force exerted by radiation, Mr. Crookes employed 

 the delicate torsion-balance already described, and 

 found the mechanical effect of the light of a can- 

 dle at twelve inches' distance, acting on two 



