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Mean (B+X)-(A+Y)=14:75 divisions = 12831 milligramme.
Greatest deviation from the mean =1:02 division =:0089 mgm.

The great difference between the result here and that in series V is probably due to deposit of dust. The new mirrors had to be fixed up just before the experiment began, and the doors were open for some time. At the conclusion of the weighing I found a good deal of dust on the weights.

VIII.—June 19.—Determination of 1 Scale Division.

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1

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Mean (B+X)-(A+Y)=47-24 divisions =0:4119 milligramme.
Greatest deviation from the mean =:43 division =

--00375

mgm.

Mgms.

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-0052} A=B+-0116 mgm.

Summary.

Greatest deviation Series.

from mean in

milligrammes. 1 (B+X)-(A+Y) •0718 .0099

A=B+0446 mgm. 2 (A+X)-(B+Y) •1610 3 (A+X)-(B+Y) 1732 4 (B+X)-(A+Y) = •1500 0122 5 (B+Y)-(A +X) = .0065 ·0057

B=A+:1989 mgm. 6 (B+X)-(A+Y) = .4043 •0078) 7 (B+Y)-(A+X) = 1283

B=A+ 1418 mgm. 8 (B+X)-(A+Y) •4119 -0037

The greatest error—that is the greatest deviation of any one value from the mean of its series-in the first four series is yoooooooth of a pound. The greatest error in the four series Nos. 5—8 is goobooth of a pound.

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II. “On Repulsion resulting from Radiation.” Part VI. By

WILLIAM CROOKES, F.R.S., V.P.C.S.

(Abstract.) In this part, with which the research closes, the author first examines the action of thin mica screens fixed on the fly of an ordinary radiometer, in modifying the movements. It is found that when a disk of thin clear mica is attached 1 millim. in front of the blacked side of the vanes of an ordinary radiometer, the fly moves negatively, the black side approaching instead of retreating from the light. When a thin mica disk is fixed on each side of the vanes of a radiometer, the result is an almost total loss of sensitiveness.

In order to examine the action of screens still further an instrument is described having the screens movable, and working on a pivot independent of the one carrying the fly, so that the screens can move freely and come close either to the black or to the white surfaces of the disks. By gentle tapping the screens can be brought within 2 millims. of the black surfaces. A candle is now brought near, shaded so that the light has to pass through one of the clear disks and fall on the black surface. The black side immediately retreats, the clear disk remaining stationary for a moment and then approaching the light. If the candle is allowed to shine on the plain side of the black disk, no immediate movement takes place. Very soon, however, both disks move in the same direction away from the candle, the speed of the clear disk gradually increasing over that of the blacked disk.

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Instead of allowing the clear screens to freely move on a pivot, an instrument was made in which the screens could be fixed beforehand in

any desired position in respect to the blacked disks. It was then found that with the screens close to the blacked sides of the vanes the fly rotates very slowly in the negative direction, stopping altogether when the candle is moved five or six inches off. With the screens 1 millim. from the black surface the direction is negative and the speed at its maximum. When the screens and disks are 7 millims. apart a position of neutrality is attained, no movement taking place. When the distance is further increased, positive rotation commences, which gets stronger as the screens approach the bright sides of the disks, where the positive rotation is at its maxi.

The author adduces reasons for considering that the negative rotations here observed are caused by the warming up of the black surface by radiation falling direct on it, through the clear mica screen, and the deflection backwards of the lines of molecular pressure thereby generated.

The action of these radiometers being complicated, owing to the surfaces of the vanes being different in absorptive power, another instruinent was made in which the vanes were of polished aluminium, perfectly flat and symmetrical with the bulb. The screens were of clear mica movable in respect to the vanes, and at right angles to their surface. When exposed to the light of a candle it was found that with the screens brought up close to the disks, the rotation was as if the unscreened side were repelled ; at an intermediate position there was neutrality. Explanations are given of these movements, but without the illustrative cuts they would be unintelligible.

Experiments on radiometers having movable screens interposed between the vanes and the bulb are next given, and these are followed by a long series of experiments on the influence of movable screens on radiometers with cup-shaped metallic vanes, the screens being varied in shape, and position in respect to the plane of rotation, as well as in respect to the distance from the vanes.

A similar series is given with metallic cylinders as vanes, and from the behaviour of the latter kind of radiometer, an explanation is given of the various movements previously obtained. It is found that when the screen touches the convex surface of the vanes the rotation under the influence of light is always positive. It commences at a low exhaustion, increases in speed till the rarefaction is so high that an ordinary radiometer would begin to lose sensitiveness, and afterwards remains at about the same speed up to the highest rarefaction yet obtained. At any rarefaction after 87 M (millionths of an atmosphere) there is a neutral position for the screen. When it is on the concave side of this neutral position the direction of rotation is positive, and when on the convex side of the neutral position it is

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