April I. 1879] 



NATURE 



511 



EXPERIMENTAL RESEARCHES ON THE 

 REPULSION RESULTING FROM RADIATION 



IN previous papers ^ I have described my earlier ex- 

 periments with the radiometer, and I then showed 

 that the movement of this instrument was due to the 

 presence of residual gas. I have since examined the 

 repulsion everted by a standard flame shining on pith 

 and mica disks, coated with various powders, chemical 

 precipitates, &c., and suspended in vacuo in a torsion 

 apparatus, and I propose in this and succeeding papers 

 to give an account of these experiments, and of the 

 concluding researches on the repulsion resulting from 

 radiation. 



The apparatus I used to get quantitative measurements 

 of the repulsion produced by radiation on various kinds 

 of disks, and coated with different substances, is similar 

 to one I have already described, but in order that the 

 experiments may be better "anderstood, it is shown in 

 Fig. I. I append the following description : — 



at ii di horizontal glass tube containing the beam, 







Fig. I. 



which, in this case, is made of straw, so as to secure lightness 

 with absence of flexure under the comparatively heavy 

 weights it sometimes has to bear ; cd is z. fine torsion 

 fibre drawn from flint glass, to which the beam is sus- 

 pended ; it is cemented at ^ to a well-ground stopper, so 

 as to admit of adjustment. When in position, cement, 

 made by fusing together eight parts by weight of resin, 

 and three parts of bees' wax, is run round the stopper. 

 At c, the point of junction between the torsion-fibre and 

 the straw beam, is a silvered glass mirror. At the end e 

 of the beam, is a small pan to hold the weights counter- 

 poising the disks, which are suspended to the other end. 

 A flat stirrup of aluminium at / fits stiffly on the straw 

 beam, and carries a flat glass fibre, / ^-j cemented to it 

 so as to allow of no play, the straw beam, the aluminium 

 hook, and the glass fibre being perfectly rigid. The 

 experimental disks are fixed on the glass fibre by means 

 * Nature, voL xv. pp. 224, 299. 



of a touch of cement at the back. The vertical tube is 

 arranged to hold six disks, the top one, h, being always 

 the same standard lamp-blacked pith ; the others, /,/, k, /, 

 and m, being changed each time. A small magnet, n, 

 attached to the central mirror, and controlled by a bar- 

 magnet outside, gives the power of bringing the beam to 

 zero, should it happen to get out of adjustment, without 

 having to melt the cement and alter the angle of the 

 torsion fibre by turning the stopper d. Plate glass caps 

 at o and p, cemented to the ground edges of the tubes, 

 give access to the interior ; allows the counterpoises to 

 be adjusted in the pan, and p allows the aluminium 

 stirrup to be unhooked, and the whole of the disks to be 

 lifted out together. The apparatus is connected to the 

 mercury pump by the arm and spiral q. The weights and 

 dimensions of the various parts of the apparatus are as 

 follows : — 



Weight of straw beam, mirror, magnetic ■ • > 



needle, alaminium stirrup, and flat glass 



fibre, &c 7'25 grains. 



Average weight of six plain mica disks ... 2*40 „ 



Average weight of six plain pith disks ... 0*59 ,,' 



Length of straw beam, from centre of coun- 

 terpoising pan to centre of disks 1 7'o centimetres. 



Length of arm from centie of suspension to 



centre of pan 7'6 ,, 



Length of arm from centre of suspension to 



centre of disks 9*4 „ 



Glass torsion fibre — Length 23 "O ,, 



,, ,, Thickness 0*0013 inch. 



Torsion with a glass weight hanging ) \ oscillation in 



from it \ 1575 seconds. 



Fig. 2 shows the apparatus fitted up for experimenta- 

 tion. The disks are shown in position at aj a brick wall, 

 be, has holes pierced through it in two places, as shown, 

 one hole, d, being opposite the centre mirror, and the 

 other, e, opposite the disks. The aperture d is lined with 

 card, lampblacked inside, and the interstices between it 

 and the bricks are well plugged with cotton wooL A 

 water cell at d prevents radiant heat from the lamp 

 getting to the apparatus. Through the hole e pass six 

 card tubes, lampblacked internally, 20 millims. diameter, 

 and 23 centims. long. The tubes are firmly cemented 

 to the wall, so that each shall be exactly central with its 

 corresponding disk, and the outer end of each is closed 

 with a cork. The space between the tubes and wall is 

 well stuffed with cotton wool. The apparatus, being 

 once fixed in position, is surrounded on all sides, as well 

 as above and below, with cotton wool. Outside this is a 

 row of glass bottles filled with water, and in front of all 

 is a wooden screen. When protected in this manner, the 

 inside of the apparatus is found to be free from disturb- 

 ances caused by changes of temperature. When the 

 disks have to be changed, air having been let in through 

 the pump, access is easily obtained to the glass cap p 

 (Fig. i), and the cement being softened by heat, and the 

 cap removed, the disks are lifted out together by seizing 

 the aluminium stirrup with forceps. A fresh set of disks 

 being introduced, the apparatus is again packed up and 

 re-exhausted. 



A lamp at / throws a narrow beam of light on the 

 mirror of the apparatus, through the aperture d. The 

 ray is reflected to the scale g, where its deflection from 

 zero shows the angular movement of the torsion beam 

 when one of the disks is repelled by radiation. The 

 scale is l|- metre from the reflecting mirror. 



A standard candle (the kind employed in gas pho- 

 tometry, and defined by Act of Parliament as a " sperm 

 candle of six to the pound, burning at the rate of I20grs. 

 per hour") is supported on a heavy stand, h, and can be 

 raised or lowered by means of the sliding piece, i. An- 

 other sliding piece, J, carries a pointed wire projecting 

 from it. The upright rod of the stand is graduated and 

 numbered, so that when the sliding piece/ is at mark i. 



