Abbildungen der Seite
PDF
EPUB

the gold leaves diverge. Whilst thus divergent I test the electric state of the electroscope, and find it negative. Hence it is obvious that a portion of its natural electricity has flown off by means of the steam. This is the simple fact, and nothing more, and gives no reason whatever why the steam should take away more fluid than naturally belonged to that portion of the water from which it was formed. By the help of Franklin's can and chain, however, we shall be enabled to arrive at a satisfactory explanation.

When the dish of water was at the common temperature it occupied a certain space, and contained its natural quantity of the electric fluid, or precisely that quantity which, in the character of water its susceptibility of charge would allow ; but as soon as it became converted into steam, its dimensions expanded, and a corresponding expansion and consequent attenuation of the electric fluid took place, in precisely the same manner as in Franklin's chain when lifted out of the can. This attenuation of electric action in the steam rendered it negative, and being in contact with the unevaporated portion of the water absorbed electric fluid from it, and thus rendered it negative.

The next step in the illustration is to shew that the vapour thus produced does absolutely carry up with it more of the electric fluid than naturally belongs to it when in the state of water. For this purpose I suspend, by means of a silken thread, a hemispherical tin vessel over the steam which rises from the water in the dish, in the manner represented in the figure. The rim of this inverted vessel is turned inwards, and formed into a channel for the purpose of collecting the water from the condensed steam. By this means I not only collect steam but its contained electric fluid also, which condenses as the steam condenses in the inverted vessel. When the evaporation and condensation has proceeded till the gold leaves of the electroscope below have diverged sufficiently for the purpose, I remove the insulated vessel to another electroscope, which immediately displays electric action ; and by the application of an excited stick of sealing-wax this action is found to be positive.

The results of these experiments are very interesting in more ways than one ; as they not only prove that steam is capable of absorbing more of the electric fluid than the water from which it is formed, but also furnish us with a satisfactory explanation of the origin of the electrization of clouds, which are well known to be masses of the condensed aqueous vapour which had ascended from the earth in a state of high attenuation. Recently, the electricity occasioned by condensing steam has been exhibited on a very ex

[graphic]

tensive scale, by the experiments of Messrs. Patterson and Armstrong, at Newcastle-upon-Tyne.*

From these considerations it would appear that all clouds at the time of their formation are electro-positive ; although, in consequence of a slowness of the cloud-forming process they may occasionally give off to the neigbouring air the redundant electric fluid almost as rapidly as it is condensed. In such cases, the resultant cloud would be neutral; but when by a sudden depression of temperature a dense cloud becomes rapidly formed, its condensed electric fluid has no time to escape in any other manner than by a sudden discharge, which is a certain result if a proper object be sufficiently near to remove it.

The object nearest to a cloud thus rapidly formed, are other clouds, either previously formed, or just coming into existence : and as it is next to impossible that any two of these clouds should be in precisely the same electric state, the positive cloud of the group darts its lightnings to those around it which are less intensely charged than itself. These again, in their turn, discharge their lightnings to others negative to themselves : and thus it is that the greatest quantity of lightning is always among the clouds, the discharges to the ground being comparatively few. It is also a remarkable fact, that when lightning does happen to strike the ground, it is generally succeeded by a profound pause of compartively long duration, during which not a glimmer of lightning is seen, nor is a murmur of thunder to be heard. Moreover, it is no unfrequent circumstance that the flash which strikes the ground terminates the electric storm.

The immensely large hailstones which frequently fall during an electric storm, even in the hottest part of the summer season, indicate a sudden depression of temperature to have taken place in the region of the clouds ; and the subsequent cold that we almost invariably experience for several successive days, would lead us to infer that, whatever may be the cause of the depression of temperature, it originated at some considerable altitude in the atmosphere, and progressed downwards to the surface of the earth.

There is not, perhaps, a more prevalent idea respecting lightning, than that the danger is over immediately the rain commences falling. This is a sad mistake : and for want of knowing better many have become victims of this terrible element. Is there a summer passes over our heads without some fatal accident from lightning ? Scarcely one. Men who are ignorant of the danger they are about to expose themselves to, and animals of all kinds, take shelter under trees, and other tall objects during an electric storm; not however from the lightning, but from the heavy rain which is falling. The tree is struck, and the ill-fated refugees

• A full description of these brilliant experiments appeared in the “Annals of Electricity, Magnetism, and Chemistry, &c.," vols. v, and vi.

killed on the spot, though prior to the fatal event no discharge had struck an object on the ground : every flash was between cloud and cloud, and the whole display in the aerial regions far above the loftiest object in the surrounding country.

That lightning strikes the ground more frequently during rain than previously is a fact that cannot be denied : and that this fact is strictly in accordance with the principles of electricity may easily be demonstrated both by analogy and experiment. An electric storm is generally preceeded by a period of dry weather, the atmosphere below the clouds being very dry, and, consequently a bad conductor, indeed it is ranked amongst the non-conductors, and the thickness of the stratum between the clouds and the ground is very considerable. But this is not the case in the region of the clouds; an immense quantity of aqueous vapour is there condensing, which renders the air a better conductor, and the distance between the clouds is but trifling. Hence, there is considerably less resistance between cloud and cloud than between the clouds and the ground; and though the difference of electric intensity might not be so great in the former, as in the latter case, the superior electric conduction, and the vicinity of the objects, tend to determine the discharges amongst themselves; and it is not till the falling rain has improved the conducting quality of the air below, and thus lessened the resistance to the electric force in the clouds, that lightning is capable of transpiercing it. It is therefore during the rain that the danger is greatest.

The experimental illustration of this topic is remarkably beautiful and satisfactory. For this purpose I employ a large Leyden jar, the universal discharger, and a plate of glass about a foot long. I bring the balls of the sliding wires of the universal discharger into contact with the glass plate whilst placed on the table, at about two inches distance from each other. One of the balls I connect with the metallic plate, and when the jar is charged to a high intensity I apply the discharging rod to connect the other with the ball of the jar : but no discharge takes place; which is in consequence of the distance between the balls on the glass being too great. I now bring the balls to about one inch from each other, get the jar up to the previous intensity, again apply the discharging rod and the discharge takes place. Now the resistance of a plate of dry air of about an inch in thickness, is nearly as much as the most intense charge of the jar is capable of overcoming. I will now moisten the air between the balls, by breathing on the glass, and I will remove the balls till they are three inches asunder. You will now see that the same extent of charge of the jar as before is capable of striking over the three inches of moist surface. I will now increase the distance between the ball to eight inches, and by means of a wet camel-hair. pencil, draw an aqueous line between them. You will now have an opportunity of viewing a most beautiful phenomenon. The discharge of the jar traverses the eight inches between the balls on

the glass, and the fluid is seen in a compact body, with all the brilliancy of lightning, passing the whole length of the aqueous line.

The striking distance, in electric language, is any distance between two bodies through which the electric fluid is capable of passing, or striking, in a compact discharge; and as the striking distance is increased by an increase of intensity of the charge, and also by reducing the resisting character of the medium, it will depend upon both of these circumstances : that is, it will be directly as the intensity, and inversely as the resisting character of the aerial medium ; which in symbols will stand thus : D is as , in which d represents the striking distance, I the intensity of the charge, and r the resisting character of the aerial medium. Hence, when the intensity is constant, the striking distance will be reciprocally as the resisting medium ; or, in still more general and familiar language, the striking distance is greater as the non-conducting quality of the air is diminished; and as the air has its non-conducting quality lessened by an admixture with water, the striking distance of lightning from a cloud in the direction of the earth must be greater during rain than when the air is not so charged with water.

Again : when the resisting medium is constantly the same, the striking distance will be as the intensity of the charge, or p is as I. Hence it is that in discharges of similar quantities of electric fluid from different sized jars, the striking distance between the ball of the discharging rod and the ball of the jar is very different, because of the difference of intensity in the two cases; the striking distance being always greatest with the smallest jar.

The subject of lightning conductors is a branch of practical electricity of exceedingly high interest, and demands the contemplation of the most profound electricians. Hitherto, however, little more has been attended to than the erection of a pointed rod of iron, without regard to situation, altitude, diameter, inferior termination, or any of those theoretical points essential to the efficacy and protection of the conductors, so as to render it a safeguard to persons and property against the most formidable element of nature.

Franklin, the inventor of lightning conductors, first proposed “for protecting houses, churches, ships, &c., from the stroke of lightning, to fix on the highest parts of these edifices upright rods of iron, made sharp as a needle, and gilt to prevent rusting; and from the foot of these rods, a wire down the outside of the building into the ground, or down round one of the shrouds of a ship, and down her sides till it reaches the water. Would not these pointed rods probably draw the electrical fire silently out of a cloud before it came near enough to strike, and thereby secure us from that most sudden and terrible mischief?

This philosopher, however, subsequently recommended continuous iron rods, of about half or three quarters of an inch diameter; which he said “may be fastened to the wall, chimney, &c., with staples of iron. The lightning will not leave the rod, a good conductor, to pass into the wall, a bad conductor, through the staples. It would rather, if any were in the wall, pass out of it into the rod, to get more readily by that conductor into the earth.

“If the building be very large and extensive, two or more rods may be placed at different parts, for greater security.

“Small ragged parts of clouds suspended in the air between the great body of clouds and the earth, often serve as partial conductors for the lightning, which proceeds from one of them to another, and by their help comes within the striking distance of the earth or a building. It therefore strikes through those conductors a building that would otherwise be out of the striking distance.

“Long sharp points communicating with the earth, and presented to such parts of clouds, drawing silently from them the fluid they are charged with, they are then attracted to the cloud, and may leave the distance so great as to be beyond the reach of striking.

“ It is therefore that we elevate the upper end of the rod six or eight feet above the highest part of the building, tapering it gradually to a fine sharp point, which is gilt to prevent its rusting. Thus the pointed rod either prevents a stroke from the cloud, or if a stroke be made, conducts it to the earth with safety to the building,

"The lower end of the rod should enter the earth so deep as to come at the moist part, perhaps two or three feet; and if bent under the surface so as to go in a horizontal line six or eight feet from the wall, and then bent again downwards three or four feet, it will prevent damage to any of the stones of the foundation."

Such were the instructions of the celebrated Franklin ; and had he recommended copper rods instead of iron, and directed them to be kept clear of the building instead of being fastened to the walls “ with staples of iron,” perhaps no better instructions could have been given; as far, at least, as an individual rod is concerned. But besides the injury that buildings may receive from a flash of lightning striking a conductor fixed close to the slates and masonry, from lateral explosions, a conductor consisting of a single branch only might be the means of drawing down destruction to some part of the building before the lightning reached the conductor ; for, were the lightning cloud on one side of the building, and the conductor on the other, the lightning would neither go round nor over the house to arrive at the conductor, unless it met with greater resistance in a direct path, and as the destination of lightning is frequently a great distance from the cloud, and its path considerably oblique, it is possible that some part of its path might be through a part of the building before it arrived at a lightning rod which formed another part of its path.

Cases of this kind have occurred, and, consequently, may possibly occur again under similar circumstances : therefore it seems to me that unless lightning conductors of proper materials and dimensions, be properly placed, they may be the means of causing the most destructive consequences to those buildings they were intended

« ZurückWeiter »