« AnteriorContinuar »
Powder not Collected in an Exhausted Receiver as in Air.
receiver until the barometric gauge was at twenty-eight inches, the powder, instead of collecting at the centre, passed across the membrane towards one side which was a little lower than the other. It passed by the middle just as it did over any other part; and when the force of the vibration was much increased, although the powder was more agitated at the middle than elsewhere, it did not collect there, but went towards the edges or quiescent parts.
Upon allowing the air to enter until the barometer stood at twenty-six inches, and repeating the experiments, the effect was nearly the same. When the vibrations were very strong, there were faint appearances of a cloud, consisting of the very finest particles, collecting at the centre; but no sensible accumulation of the powder took place.
At twenty-four inches of the barometer the accumulation at the centre began to appear, and there was a sensible, though very slight, effect visible of the return of the powder from the edges.
At twenty-two inches these effects were stronger; and when the barometer was at twenty inches, the currents of air within the receiver had force enough to cause the collection of the principal part of the powder at the centre of vibration.
Upon again restoring the exhaustion to twenty-eight inches, all the effects were reproduced as at first, and the powder again proceeded to the lower or quiescent parts of the membrane.
h. In denser media than air, as in water, for instance, heavier powder, as sand and filings, performs the part of light powders in air, and is carried from the quiescent to the vibrating parts.
162. The currents may be obstructed by pieces of card, and directed to points different from those of greatest excursion.
a. If a piece of card, about an inch long and a quarter of an inch wide, is fixed by a little soft cement on the face of the square plate, used in the preceding article, near one edge, the plate held as before at the middle, fine powder strewed upon it, and the bow applied at the middle of another edge; the powder immediately advances
The Currents Obstructed by Pieces of Card.
close to the card, and the place of the cloud is much nearer the edge than before. Fig. 139 represents the arrangement; the diagonal lines being those which sand would have formed, the line at the top, a, representing the place of the card, and the X to the right, the place where the bow was applied; b represents the place of the cloud when no card is present, and it does not proceed to the point of greatest excursion at the very edge of the plate, on account of the manner in which the air is there agitated.
b. If pieces of card are fixed on the glass in the three angular forms represented in fig. 140, upon vibrating the plate the fine powder always goes into the angle, notwithstanding its difference of position in the three experiments, but perfectly in accordance with the idea of currents intercepted more or less by the card.
c. When two pieces of card are fixed on the plate, as in fig. 141, a, the powder proceeds into the angle, but not to the edge of the glass, remaining about th of an inch from it; but on closing up that opening, as at b, the powder goes quite up into the corner. Upon fixing two pieces of card on the plate, as at c, the powder between them collects in the middle, very nearly as if no card had been present; but that on the outside of the cards gathers close up against them, being able to proceed so far in its way to the middle, but no further.
d. When a long glass plate is supported by bridges or strings at the two nodal lines, represented in fig. 142, and made to vibrate, the powder collects in three divisions. That between the nodal lines does not proceed at once into a line equidistant from the nodal lines and parallel to them, but advances from the edges of the plate towards the middle by paths, which are a little curved and oblique to the edges where they occur near the nodal lines, but are almost perpendicular to it elsewhere; and the powder gradually forms a line along the middle of the plate; it is only by continuing the experiment for some time, that it gathers up into a heap or cloud equidistant from the nodal line.
The Currents Obstructed by Pieces of Card.
But upon fixing card walls upon the plate, as in fig. 143, the course of the powder within the cards is directly parallel to them and to the edge, instead of being perpendicular, and also directly towards the point of greatest excursion.
To prove that it is not as a weight that the card acts, but as an obstacle to the currents of air formed, it is bent flat down outwards, without being moved from its place, and then the powder resumes the courses it took when without the cards.
The powder sprinkled over the extremities of the plate proceeds towards places equidistant from the sides and near the ends, as at a, fig. 144; but on cementing a piece of paper to the edge, so as to form a wall about one quarter or one third of an inch high, b, the powder immediately moves up to it, and retains this new place.
In a longer narrow plate, similarly arranged, the powder can be made to pass to either edge, or to the middle, according as paper interceptors to the currents of air are applied.
163. The air, carried forward by the currents to the points of greatest excursion, rises from the vibrating surface at these points, proceeds to a greater or less distance from the surface, and then returns to the nodal lines, forming currents in opposite directions to the first, a little above them, and blending more or less with them.
a. Mr. Faraday endeavoured, in various ways, to make the extent of this system of currents visible. In the experiment already referred to, where gold-leaf was placed over the point of greatest excursion, the upward current at the most powerful part was able to raise the leaf about one tenth of an inch from the plate. The clouds formed at the points of greatest excursion are always higher and larger, as the vibrations are stronger, and may frequently be seen rising up in the middle and flowing over towards the sides.
System of Currents in the Vibrations of Surfaces.
b. In the receiver of an air-pump, if the powder collected at the middle of the circular stretched membrane is observed, as to the height to which it is forced upwards by the vibrations; and then the receiver being exhausted, if the height to which the powder is thrown by similar vibrations is again observed; in the latter case it is nothing like so great as in the former, the height not being two-thirds, and barely one-half, the first height. Were the powder thrown up by mere propulsion, it should rise far higher in vacuo than in air; but the reverse takes place, because that in air the current has force enough to carry the fine particles up to a height, far beyond what the mere blow which they receive from the vibrating membrane can effect.
c. When the circular stretched membrane is made to vibrate, and a large glass tube, as a cylindrical lamp-glass is brought near to its centre; the most striking proofs are obtained of the existence of carrying currents by the effects upon the light powder, which flies rapidly under the edge, and tends to collect towards the axis of the tube; it may even be diverted somewhat from its course towards the point of greatest excursion. A piece of upright paper, held with its edge equally near, does not produce the same effect; but immediately that it is rolled into a tube, it does.
When the glass chimney is suspended very carefully, and at but a small distance from the membrane, the powder often collects at the edge, and revolves there; a complicated action between the currents and the space under the thickness of the glass taking place, but still tending to show the influence of the air in arranging and disposing the powder.
d. Let a sheet of drawing-paper be stretched tightly over a frame, so as to form a tense elastic surface nearly three feet by two in extent. Upon placing this in a horizontal position, throwing a spoonful of lycopodium upon it, and striking it smartly below with the fingers, the phenomena of collection at the point of greatest excursion, and of moving heaps, can be obtained upon á magnificent scale. When the lycopodium is uniformly spread over the surface, and any part of the paper slightly tapped by the hand, the
Influence of Tubes and Plates held over Vibrating Surfaces under the Currents.
lycopodium at any place chosen can be drawn together merely by holding the lamp-glass over it. It will be unnecessary to enter into the detail of the various actions combining to produce these effects; it is sufficiently evident, from the mode in which they may be varied, that they depend upon currents of air.
e. A very interesting set of effects occur, when the circular stretched membrane is vibrated under plates; the powder collects with much greater rapidity than without the plate; and instead of forming the semi-globular moving heaps, it forms linear arrangements, all concentric to the centre of the membrane.
When the vibrations are strong, these assume a revolving motion, rolling towards the centre at the lower part in contact with the membrane, and from it at the upper part nearest the glass; thus illustrating in the clearest manner the double currents caged up between the glass and the membrane.
f. Sometimes, when the plate is held down very close and tight, and the vibrations are few and large, the powder is all blown out at the edge; for then the whole arrangement acts as a bellows; and as the entering air travels with much less velocity than the expelled air, and as the forces of the currents are as the squares of the velocity, the issuing air carries the powder more forcibly than the air which passes in, and finally throws it out.
164. When a surface, vibrating transversely, is covered with a layer of liquid, that liquid, if of sufficient tenacity, is determined from the quiescent to the vibrating parts, producing accumulation at the latter places; but this accumulation is limited, so that, if purposely rendered too great by gravity or other means, it will quickly be diminished by the vibrations, until the depth of fluid at any one part has a certain and constant relation to the velocity there, and to the depth elsewhere.