« ZurückWeiter »
COMMUNICATION & VELOCITY OF ELECTRICITY. 275
the operator, when making these experiments with large jars ; for it is not to be doubted, but that several of these fully charged would as certainly, by increasing them, in proportion to the size, kill a man, as they did before the turkey.
[Phil. Trans, 1751.
1. Communication and velocity of Electricity.
By W. Watson, Esq.
In a former paper Mr. W. took notice, that among the many surprising properties of electricity, none was more remarkable, than that the electrical power, accumulated in any non-electric matter contained in a glass phial, described on its explosion a cir. cuit through any line of substances non-electrical in a considerable degree; if one end of it was in contact with the external surface of this phial, and the other end on the explosion touched either the electrified gun barrel, to which the phial in charging was usually connected, or the iron hook always fitted in it.
This circuit, where the non-electric substances, which happen to be between the outside of the phial and its hook, conduct electricity equally well, is always described in the shortest route possible ; but if they conduct differently, this circuit is always formed through the best conductor, how great soever its length is, rather than through one which conducts not so well, though of much less extent.
It has been found, that in proportion as bodies are susceptible of having electricity excited in them by friction, in that proportion they are less fit to conduct it to the other bodies; in consequence, of all the substances we are acquainted with, metals conduct best the electrical powers; for which reason the circuit before spoken of is formed through them the most readily. Water likewise is an excellent conductor; for the electrical power makes no difference between solids and fluids as such, but only as they are non-electric matter.
Mons. le Monnier the younger, at Paris, in an account transmitted to the Royal Society, takes notice of his feeling the stroke of the electrified phial along the water of two of the basins of the Thuilleries, the surface one of which is about an acre, by means
of an iron chain which lay on the ground, and was stretched round half their circumference. On these considerations it was conjectured, as no circuit had as yet been found large enough so to dis. sipate the electrical power as not to make it perceptible, that if the Don-electrical conductors were properly disposed, an observer might be made sensible of the electrical commotion quite across the river Thames, by the communication of no other medium than the water of that river. In any other part of natural philosophy, as we should draw conclusions only from the facts themselves, it was determined to make the experiment.
The making this experiment drew on many others, and as the gentlemen concerned flatter themselves that they were made with some degree of attention and accuracy, they thought it not improper to lay a detail of all the operations before the Royal Society. To try this experiment, it was absolutely necessary that a line of non-electric matter, equal in length to the breadth of the river, should be laid over it, so as not to touch the water in any part of its length; and the bridge of Westminster was thought the most proper for that purpose, where the water from shore to shose was somewhat more that four hundred yards.
Accordingly on July 14, 1747, several members of the Royal Society met to assist in making the experiment. A line of wire lail along the bridge, not only through its whole length, but likewise turning at the abutments, reached down the stone steps on each side of the river low enough for an observer to dip into the water an iron rod held in his hand. One of the company then stood on the steps of the Westminster shore, holding this wire in his left hand, and an iron rod touching the water in his right ; on the steps facing the former on the Surry shore, another of the company took hold of the wire with his right hand, and grasped with his left a large phial almost filled with filings of iron, coated with sheet-lead, and highly electrified by a glass globe properly disposed in a neigh
bouring house. Athird observer standing near the second dipped ap iron rod held in his left hand into the water, and touching the iron hook of the charged phial with a finger of his right hand, the electricity snapped, and its commotion was felt by all the three observers, but much more by those on the Surry shore. The third observer here was no otherwise necessary, than that the river being full, the iron was not long enough to be fixed in the mud on
the shore, and therefore was in wast of some support. riment was repeated several times, both then and afterwards, and electrical motion felt across the river. The length of this circuit, through which the electricity was propagated, was at least eight hundred yards, more than four hundred yards of which was formed by the stream of the river.
The observers on the Westminster shore not feeling the electric cal the commotion equally strong with those of Surry, was judged to proceed from other causes besides that of distance. For it must be considered, that the conducting wire was almost throughout its whole length laid on Portland stone standing in water. This stone being in a great degree non-electric, is of itself a conductor of electricity; and this stone standing in water, no more of the electri. city was transmitted to the observers on the Westminster shore than that proportion, on which iron is more non-electric, and con. sequently a better conductor of electricity than stone. Whether the conducting wire on the bridge was broken or no, and, conse. quently, whether the observers on the Westminster shore felt the electrical commotion or no, not only the observers on the Surry shore, who with their wire formed part of the line, felt the shock in their arms; but those persons who only stood on the stone steps there, and touched the wire with their fingers, felt the electrical commotion in the arm of that hand which touched the wire. Hence, and from a person feeling the electrical commotion stand. ing on the wet stone steps of the Westmioster shore, though not forming part of the line, but only touching the wire with his fin. gers, it was concluded, that besides the large circuit before spoken of, there were formed several other subordinate circuits, between the same steps of the Surry shore, and the bridge by means of the water; by which that part of the electrical power, felt by the ob. servers on the Surry side of the river, and not by those on the Westminster side, was discharged.
Dr. Bevis having observed, that which was likewise tried here, that however well an electrified phial was charged, its iron hook would not fire the vapours of warm spirits of wine held in a spoon and applied to it, if the person who held the phial, and who held the spoon, did not take each other by the hand, or have some other non-electrical communication between them; it was there. fore thought proper to try the effects of electricity on some warm spirit of wine through the large circuit before mentioned. Ac. cordingly the observers being placed as before, both on the West. minster and Surry shores, no other alteration was made in the be. fore mentioned apparatus, than that the wire which connected the gun-barrel with the iron hook of the coated phial being laid aside, the coated phial itself was charged at the gun-barrel, and then brought in the hands of an observer near the warm spirits in the spoon, which was placed on the short iron rod beforementioned, which was connected with the wire which went to the observers on the Surry shore. On presenting properly the iron hook of the charged phial to the warm spirit, it was instantly fired, and the elec. cal commotion felt by the observers on both sides of the river.
It was then thought proper to try the effects of the charged phial on the warm spirit, when the wire was divided which was laid over the bridge; on presenting the iron hook to the spirit, a sufficient snap was given to the spoon to fire the spirit, but nothing so smart as in the former experiment where the large circuit was completed.
It was then tried what effect would be on the spirit, if the charg. ed phial was divested of its long wire which lay over the bridge, and was only held in the hand of an observer ; while the spoon with warm spirit was placed in contact of the iron rod beforemen. tioned, to which the wire was connected, which went to the obser. vers on the Surry shore ; and the spirit was fired with much the same degree of smartness as in the last experiment.
In these and all the subsequent operations, wires were made use of to conduct the electricity preferable to chains, as by great numbers of experiments it had been fully proved, that whatever difference there was in the bulk of the conductor, viz. whether it were a small wire, or a thick iron bar, the electrical strokes communi. cated were equally strong; and it had been further observed, be. sides the difficulty of procuring chains of a requisite length for the present purposes, that the stroke at the gun-barrel, when the electricity was conducted by a chain, was cæteris paribus not so strong, as when that power was conducted by a wire. This was occasioned by the junctures of the links of the chain not be. ing sufficiently close, which caused the electricity in its passage to snap and flash at the junctures, where there was the least separation; and these smaller snappings in the whole length of the chain lessened the great one of the gun-barrel.
Encouraged by the success of these trials, the gentlemen were desirous of continuing their inquiries, and of knowing whether the electrical commotions were perceptible at a still greater distance. The New River near Stoke Newington was thought most coveni. ent for that purpose ; as at the bottom of that town, the windings of the river are such, that from a place which he calls A to another B, the distance by land is about eight hundred feet, but the course of the river is near two thousand. From A to another place C, in a right line is two thousand eight hundred feet, but the course of the water is near eight thousand feet.
Accordingly, on Friday July 24, 1747, there met at Stoke Newington the president of the Royal Society and several other gentlemen : when every being thing disposed as before, and the wire extended from A to B and C, over the meadow, without touching the water. When every thing was thus disposed, and the signals given, the charged phial was exploded several times, and the electrical commotion every time smartly felt by the observers both at A and B. In the like trials with the places A and C, the commotions were perceptible from A to C; a distance not less than two thousand eight hundred feet by land, and near eight thousand by water.
To execute this, to the former wire, which was already conduct. ed to B, another was added, which there crossed the river with. out touching the water; and reached almost to C, where the first of a line of gentlemen held as before the wire in one hand, and the last dipped the iron into the water. The wire from the ma. chine to A was as before. Its effects were plainly though but faintly perceived each time by some of the observers, but never by them all. The electrical commotion was always felt by that observer, who held the extremity of the wire, but never by him who held the iron rod in the water. It was in one experiment felt by the observer wbo held the wire, not felt by the next who held the hand of the former, and yet plainly perceived by the third who joined the second. Those who did not themselves feel the electri. cal commotion here, did as at B, see the involuntary motions of those who did. The observers at A felt the shocks in the same degree, whether the other observers were stationed at B or C.
This experiment further demonstrates the distance to which the electrical power may be conveyed : but the same difficulty occurs here as in the last, viz, whether the circuit was completed by the water of the river, or by the ground which was wet?