« ZurückWeiter »
this seems to depend very little on their specific gravity, simply considered: metals for instance, and water, are in a great degree non-electrics, and consequently conduct electricity the best of any substances, that have yet fallen under our notice; whereas the calxes of metals, though very dense bodies, and very greatly more so than water, prevent in a great degree the quick propagation of the electrical power. So that a phial coated within and without with ceruse, i. e. the calx of lead, and electrised, did not, on the application as usual of one hand to the external surface, and touching the prime conductor with the other, occasion any shock, or make any explosion, more than the simple stroke from the prime conductor. The same observation holds good with regard to red lead, litharge, and lunar caustic or the calx of silver, none of which snap when electrised. For the same reason, filings of iron, which are rusty, i. e. have their surfaces converted into a calx, are much less proper to be put in glasses to make the Leyden experiment, than those that are not; inasmuch as these last cause a much louder explosion than the first.
Mr. W. procured a glass jar as large as possible, so that the glass might be very thin; the height of which was twenty-two inches, the periphery forty-one. This was covered within and without, leaving a margin of an inch at top, with leaf-brass. As much of the internal surface as was covered amounted to 1129 square inches. But the difficulty he met with in procuring this glass, was sufficiently recompensed by the great increase of the explosion from it, when fully electrised, and discharged in the same manner as before. The report was vastly louder; all the attendant phænomena greatly exceeded any thing of this kind he was before acquainted with. As the quantity of metal within this jar did not exceed two drams, this experiment gives further weight to his opinion in regard to the manner of increasing the effects of the Leyden ex. periment; and from what the phænomena of that surprising expe riment principally proceed; viz. not from the volume of the prime conductor, nor from the quantity of non-electrical matter contained in the glass, but from the number of points of non-electrical contact both within and without-side of the glass, and from the density of the matter constituting those points. It must be observed that cæteris paribus, the electrical explosion is greater from hot water included in glasses, than from cold; and from these glass jars warmed, than when they are cold.
The explosions from the large glasses just mentioned fully electrised, as well as from small ones under the same circum stances, will not be considerable, unless the circuit be completed; that is, unless some matter, non-electric in a considerable degree, and in contact with the coatings of the phials, is brought into contact, or nearly so, with such non-electrics as communicate with When indeed the the matter contained in the phials themselves.
circuit can be completed, the explosion from the large glasses is prodigious; the whole quantity of electricity accumulated, or nearly so, being discharged in an instant. But the fact is otherwise if the circuit is not completed, and the iron rod in the mouth of one of these phials is touched by a non-electric (the hand of a man for instance) not in contact with the tail wire: for then there will be no explosion, no shock; but the person approaching his finger near the iron rod, will see a succession of small sparks, more intensely red than that large one seen when the phials explode at once; and the person making the experiment will feel a very pungent pain, but confined to that finger which touches the iron rod. This succession of sparks continues till the electricity accu. mulated in the phials is nearly exhausted. So that the explosion from any given quantity of electricity, accumulated as beforementioned, is greater or less in proportion to the time expended in making that explosion: in like manner as a given quantity of grained gunpowder rammed hard in a pistol, is almost instantaneously fired, and that with a great report; when the same quantity of gunpowder rubbed fine, and rammed hard, takes a considerable time in burning as a squib, and makes no explosion.
From what he has advanced, it may possibly be conjectured, that the electrical effluvia occupy only the surfaces of bodies electrised; as we found that a very small body of matter, distributed under a very large surface, would occasion a greater accumulation of electricity, than a much more considerable quantity of matter under a less. But that the electricity occupies the whole masses of bodies electrised, and passes through their constituent parts, Mr. W. thinks is clearly demonstrated by the following experiment.
He enveloped an iron rod, about three feet in length, with a mixture of wax and resin, leaving free from this mixture only one inch at each end. This iron was warmed, when thus fitted, that the whole of its surface where it was intended might be covered,
This rod, when electrised at one of its ends, snapped as strongly 'at the other, as though it was without wax and resin. This could not have happened from the electricities passing along the surface of the iron rod, because there it was prevented by the originally. electrics. and consequently must of necessity pass through it.Again, a phial of water in the experiment of Leyden can be elec. trised, and may be caused to explode, though the wire, touching the water in the phial in making that experiment, be run through a wax stopple, exactly fitted to the mouth of the phial.
He caused a glass tube, open at each end, and about two feet and a half long, to be capped with brass cemented to the ends of the tube. In the centre of each of these caps was fastened a slen. der brass rod; and these were disposed so in the tube, as to come within half an inch of each other. When the tube was properly suspended in silk lines, with one of its extremities neara glass globe in motion, the brass work at both ends snapped equally strong. As the 'electricity could not pass along the surface of this tube warmed and 'wiped clean, this effect could not have happened, unless the elec. tricity pervaded the substance of the brass caps. On touching the brass at the end of the tube most remote from the electrifying ma'chine, the snaps from one of the brass rods within the tube to the other were seen to correspond with the snaps without. More ex. periments of this kind might be added, but these he presumes are 'sufficient to show, that the electricity occupies the whole masses of non-electric bodies electrised.
Mr. W. mentions a series of experiments he had made in vacuo; from the comparison of which with the experiments in open air, it appears that our atmosphere, when dry, is the agent by which, with the assistance of other electrics per se, we are enabled to ac. cumulate electricity in and upon non-electrics; that is, to com. municate to them a greater quantity of electricity than they natu rally have: hence also we see, that on the removal of the air, the electricity pervades the vacuum to a considerable distance, and manifests its effects on any non-electrics, which terminate that va. cuum ; and by these means that originally-electric bodies, even in their most perfect state, put on the appearance of non-electrics, by becoming the conductors of electricity.
[Phil. Trans. Abr. 1747.8.
2. The same Subject continued.
By the same.
MR. Watson laid before the Royal Society* an account of what had been done by some gentlemen, in order to ascertain the respective velocities of electricity and sound; from which it appeared, that through a space measuring 6732 feet, the electricity was perceptible in a quantity of time less than of a second. But the gentlemen concerned were desirous, if possible, of ascertaining the absolute velocity of electricity at a certain distance; and a me. thod had been thought of, by which this might be determined with great exactness.
Accordingly, August 5, 1748, there met at Shooter's Hill for this purpose, the president of the R. S. the Rev. Mr. Birch, the Rev. Dr. Bradley, astronomer royal, James Burrow, Esq. Mr. Ellicot, Mr. George Graham, Richard Graham, Esq. the Rev. Mr. Lawrie, Charles Stanhope, Esq. and Mr. W. who were of the Royal Society; also Dr. Bevis, and Mr. Grischow, jun. a member of the Royal Academy of Sciences at Berlin.
It was agreed to make the electrical circuit of two miles; in the middle of which an observer was to take in each hand one of the extremities of a wire, which was a mile in length. These wires were to be so disposed, that this observer being placed on the floor of the room near the electrical machine, the other observers might be able in the same view to see the explosion of the charged phial, and the observer holding the wire; and might take notice of the time lapsed between the discharging the phial and the convulsive motions of the arms of the observer in consequence of it; as this time would show the velocity of electricity, through a space equal to the length of the wire between the coated phial and the observer.
The electrifying machine was placed in the same house as it was last year. We then found ourselves, says Mr. W. greatly embarrassed by the wire's being conducted by the side of the road, which we were compelled to, on account of the space necessary for the measuring of sound; but so great a distance from the machine was
See the preceding article.
296 COMMUNICATION & VELOCITY OF ELECTRICITY.
not now wanted, though the circuit through the wire was intended to be at least two miles. We had discovered, by our former experiments, that the only caution now necessary was, that the wires conducted on dry sticks should not touch the ground, nor each other, nor any non-electrical in a considerable degree, in any part of their length; if they did not touch each other, the returns of the wire, be they ever so frequent, imported little, as the wire had been found to conduct electricity so much better than the sticks. It was therefore thought proper to place these sticks in a field fifty yards distant from the machine. The length of this field being eleven chains, or 726 feet, eight returns of the wire from the top to the bottom of the field, made somewhat more than a mile, and sixteen returns more than two miles, the quantity of wire intended for the electricity to pass through to make the experiment.
We had found last year, that on discharging the electrified phials, if two observers made their bodies part of the circuit, one of which grasped the leaden coating of the phial in one hand, and held in his other one extremity of the conducting wire; and if the other ob. server held the other extremity of the conducting wire in one hand, and took in his other the short iron rod with which the explosion was made; on this explosion, they were both shocked in the same instant, which was that of the explosion of the phial. If therefore an observer, making his body part of the circuit, was shocked in the instant of the explosion of the charged phial in the middle of the wire, no doubt would remain of the velocity of electricity being instantaneous through the length of that whole wire. But if, on the the contrary, the time between making the explosion, and seeing the convulsions in the arms of the observer holding the conducting wires, was great enough to be measured, we then should be able to ascertain its velocity to the distance equal to half the quantity of wire employed only, let the manner of the electricity's discharging itself be what it would.
To make the experiment, the same phial filled with filings of iron, and coated with sheet-lead, which was used last year, was placed in the window of the room near the machine, and was connected to the prime conductor by a piece of wire. To the coating of this phial a wire was fastened; which being conducted on dry sticks to the before-mentioned field, was carried in like manner to the bottom; and being conducted thus from the bottom of the field