« ZurückWeiter »
It appears singular, at first sight, to observe negative points which are receiving electric fluid from the air, repel the uninsulated ball as decidedly as those positive points which throw the fluid off; but the immediate cause is the same in both cases, for it can be shown that a current of air proceeds from the negative point also. The air contiguous to the side of the wire is attracted more and more towards the negative point, where it deposits its electric fuid, and asterwards driven off by succeeding portions, which in their turns are driven off also; hence a continual wind is kept blowing from the negative point. The opposite figure will give some idea of the manner in which the electric fluid would rush out of a positive point through the air to the negative wire.
If instead of a metallic point we were to employ water within a capillary tube, that water would be thrown out in a divergent stream similar to the aura in the air. For this purpose we employ a small metal bucket with a capillary tube inserted in the bottom. When this bucket is partly filled with water, small drops occasionally fall from the lower orifice of the tube; but if it be hung on the prime conductor, as represented by the cut, and the machine in action, the water flows copiously in a divergent stream, as represented in the figure. When the room is darkened the divergent current is slightly luminous.
In a previous lecture I promised to bring forward some of those contrivances which have been invented for the purpose of measuring the quantity of electric Auid constituting the charge of Leyden jars.
The only instrument for this purpose that has gained any celebrity amongst writers on electricity, is called the unit jar : and as we have one of these unit jars Bbelonging to the Institution, I will describe its structure and point out its principles of action with some degree of minuteness. It is represented opposite.
This celebrated instrument is formed of a small cylindrical Leyden jar, supported in an inverted position on a glass stem, well covered with lacvarnish, and fixed into a wooden foot, as seen in the figure. The inner coating of the jar is in metallic contact with a brass ball and wire D, A; ano. ther ball above the former is in contact with the outer coating of the jar, hy means of a metallic frame and sliding wire. The brass ball g, and wire B, are also connected with the outer coating. The arrangement of this apparatus, is obviously the same as that of the medical jar, with an at
tached Lane's discharger, for is the wire A D be connected with the prime conductor, and the wire B with the ground, the charge will proceed till the resistance between the ball D, and that above it is overcome; which accomplished, the jar will discharge spontaneously; and so long as this resistance is constant, and the outside surface uninsulated, similar quantities of fluid will cause corresponding discharges. Now as the spontaneous discharges take place between the two fixed balls, the striking distance is constant, and the discharge through that striking distance will depend upon the intensity, and not upon the quantity of fluid in the unit jar. For convenience we will call this requisite intensity the discharging intensity ; which would be constantly the same if the striking distance presented a constant resistance; although the quantity of fluid required for this discharging intensity might vary considerably, according to the facility afforded for displacement of the fluid from its outer surface.
If, for instance, we have two jars, wbose figure and extent of coated surfaces were precisely the same, but the thickness of the glass considerably different, that made of the thinner glass would require much more fluid than the thick one, to arrive at any given intensity of charge ; because of the latter offering a greater resistance to the disturbing force of the accumulated fluid within; and if the resistance were augmented by any means whatever, the standard intensity would be arrived at by a still less quantity of electric fluid. Such are the considerations to be attended to in explaining the operations of the unit jar.
When this instrument is used with a view of measuring the quantity of electric fluid which charges another jar, in all cases much larger than itself, and which for convenience we will call J, the wire G B is connected with its inside, and the wire A D with the prime conductor. When the machine is put into motion, the unit jar charges; a portion of the fluid belonging to its outside being driven into the inner surface of J, which, consequently, to a certain extent charges also by polarization. Now the quantity of fluid driven into the unit jar will depend upon the quantity driven into the jar J, and the quantity driven into c, will depend on the thinness of its glass; therefore, the first unit of Auid for the discharging intensity depends upon the substance of glass of the jar J, and however thin that glass may be, the discharging intensity will require less fluid in the unit jar than when its coating was connected with the ground.
We now suppose that the first discharge has taken place, and that nearly an unit of fluid is thus thrown into j (the whole could not be thrown in because of the initial part of the discharge partially charging the outer surface in common with the inside of J). The resistance of s against the reception of fluid from the outside of the unit jar is now increased, and the discharging intensity will be accomplished by a less quantity of Auid than at first : and this second discharge of the unit jar throws a still less proportion of the diminished quantity into į than in the previous discharge. And
thus it is that each succeeding charge requires less and less fuid for the discharging intensity, and a corresponding disproportion enters the jar j.
When the intensity of 3 becomes considerable, the unit jar will be nearly choaked up, and incapable of receiving any but a very trifling quantity of fluid; and were the resistance of the striking distance not altered during this time, the discharges from the ball D to that above it would take place as if no upit jar were there. This resistance, however, is increased, which requires a higher discharging intensity, and consequently somewhat more fluid than if no increase of resistance had taken place. But this increase of resistance in the striking distance is that which lessens the quantity discharged, which at high intensities of the jar v is very small indeed.
The electrical sportsman is another piece of apparatus which operates upon the principles of Lave's discharging electrometer. The opposite figure represents this apparatus, which consists of a Leyden jar, a flock of birds, and the sportsman with his gun. From the inside of the jar proceeds a long bent brass wire, with a small wooden stage near its remote end. To the extremity of the wire four or five me content updates and interesante che fanno mai , BALI TER threads are tied having artificial birds at their other ends, which rest on the stage. Another wire, · terminated with a small ball, also rises a short height from the inside
of the jar. The outside of the jar is connected with the table and also with the gun, at the end of which is a small ball, and brought to within striking distance of the ball of the jar.
The inside of the jar being counected with the prime conductor, and the machine put into motion, the charge proceeds, and at the same time the birds rise and fly from one another by repulsion, until the striking intensity discharges the jar to the muzzle of the piece, when a flash is seen, and the birds drop as if shot by the discharge.
Whilst on the subject of electrized glass, I will offer to your notice a few curious experiments on flat glass plates. I will first operate with the glass discs, and the two metal discs before alluded to. I place the glass plates between the two metallic piates, and charge the upper side positively by uniting it with the prime conductor whilst the lower surface is uninsulated. This doue I remove the connections with the prime conductor and the table, and then discharge the glass plate by an application of the discharging rod. When the discharging rod has been removed I take up the upper metal plate by its glass handle, and by applying it to my knuckle I receive a feeble spark. I now replace this plate on the glass, and with one hand I touch the other plate : on approaching the upper plate with the other hand I again experience a spark. I
now lift up the upper plate by its glass handle aud experience another spark on presenting the knuckle, in precisely the same manner as with the electrophorus. But what is very remarkable, the power of these sparks increases to a great extent, and again diminishes, and so on for a long time together.
I now employ two square glazs plates, each of which is coated on one side only; I place the naked surfaces upon one another, and press them close together. This done, one of the coatings is connected with the prime conductor, and the other with the ground: put the machine in action, a charge takes place, and the two plates are held together by a great force. Indeed, it is difficult to separate them without fear of breaking one or both. Having accomplished their separation, and applied their surfaces to the electroscope, I find both sides of one of the glass plates positive, and both sides of the other negative. The two plates when together had obviously operated as one plate only. I now again put them together as before, and by applying the discharging rod to the two coatings a discharge takes place, and the plates easily separate.
I charge the two plates when again in close contact, and on trial find they are held fast together. I now turn the negative side upwards, and connect it with the prime conductor, and a moment's action of the machine neutralizes the electrization of the plates, and they are easily separated; but if the action of the machine be continued too long, the plate becomes electrized the reverse way, and are held together as before.
This is an old experiment, first made known by the father Jesuits at Pekin to the academy at St. Petersburg, in the year 1755. It was extensively investigated by Mr. Symmer in this country, and by M. Cigna and Father Beccaria in Italy, who were led to several other interesting experiments in consequence.
To insure success it is necessary that the glass plates have perfectly flat surfaces, and by being square the corners of the one can be placed across the sides of the other, thus giving a better opportunity of separating them when charged.
Mr. Symmer made a great number of experiments with black and white silk stockings, which, when one of each colour was worn on the same leg for half an hour, and both taken off together without separating them, showed but feeble signs of electric action; but on separating them afterwards they were found to adhere together with great force, and a crackling noise was heard and sparks seen all the time. When quite separated, and one held up in each hand, the repulsion in each stocking was so powerful that it stood out in full shape as if the leg were in it. The attraction between the two stockings was powerful, and a spark was seen as they rushed together. When one stocking was in the other, a force of fifteen pounds was required to separate them. The white stocking was always electro-positive.
The experiments with the two glass plates lead to the explanation of another curious fact attending charged glass generally. When a
plate of glass, coated on both sides, is charged to a high intensity, and then brought over the cap of an electroscope, the gold leaves are but little affected whichever surface be turned towards the instrument, although several sparks have been thrown on the positive surface, each of which if thrown on a metal surface of the same extent would have affected the electroscope to a greater degree. The glass is intensely charged, but its accumulated fuid on the positive side presses inwards to a much greater extent than outwards, because of the negative surface offering a less resistance than the pressure on the other side; and the forces are principally engaged in the substance of the glass, there being but a small portion disengaged to operate on external bodies. If, however, either of the coated surfaces be uninsulated for a moment, a portion of the previously engaged force is relieved, and the opposite side of the glass now affects the electroscope much more than before. This doctrine is beautisully illustrated by operating on an insulated Leyden jar, furnished with a pith-ball electroscope on each surface, as represented in the figure. When the jar is charged, and removed from & the prime conductor for a few moments, both pairs of balls diverge a little, but not much, for reasons already stated. I vow touch the outer coating, and its balls immediately collapse, being uninsulated; but the other pair of balls diverge to a great extent, because my finger has supplied a small portion of electric fluid to the outer surface, and relieved a corresponding quantity on the inner surface, which now being disengaged in the glass, springs to the balls which it repels. · I now take away my finger from the outer surface, and place it on the ball belonging to the inner one. By this means I take away the previously released fluid, and also a little more; and the balance of forces through the glass is again broken, and the negative balls diverge in quest of that portion last taken away from the inside. I again change the position of my finger, and the outside balls collapse, and the inside pair diverge as at first; then by a series of contacts with my finger, first with one coating and then with the other, I gradually discharge the jar.
There are several other methods of discharging a jar gradually and silently, but none more effectually than by a pointed wire. II, for instance, the ball were removed from the stem of a jar it exposes a sharp poiut, and on this point being applied to the prime conductor the charge is accomplished in the usual way: now remove the jar and place it on the table, and its charge will issue from the point into the air; and in a short time the jar will become perfectly neutral. When this experiment is made in a darkened room, a beautiful purple brush of light is seen issuing from the point, and by close attention the brush appears largest and brightest at first, and gradually