Abbildungen der Seite
PDF
EPUB

During the summer season it is always difficult, if not dangerous, to elevate the kite when clouds are about, without the precaution of first stretching the string on the ground and making the other preparations already named : for want of such precaution I have frequently experienced severe blows whilst paying out the string. When there is any appearance of lightning, even though not near, the string must never be let pass through the hand whilst elevating the kite. Flashes of lightning invariably produce electric waves in the air to a great distance on every side, and these waves produce tremendous discharges through the medium of the kite string when it happens to be in their way, and might injure or even kill the operator were he close to the apparatus at the time.

Floating clouds also, when no lightning is present, are invariably productive of electric waves, when they are highly charged. I have had a good deal of experience amongst electric waves thus produced, and occasionally have permitted others to experience their effects.* I have frequently been much annoyed by powerful shocks from waves whilst taking in the kite string. On one occasion I was struck a violent blow by a discharge which passed over about two yards of silk ribbon, though the kite string was uninsulated at the time, being tied to a tree. The cloud producing the wave was thin and of small dimensions, and not within a quarter of a mile of the kite.

To give an idea of the manner in which electrical waves are formed, and of their influence on bodies amongst which they flow, it will be necessary to call to your recollection the illustrations already given on the subject of electro-polarization. You are aware that an already charged body has the power of disturbing the natural electric equilibrium of those bodies which are placed within its sphere of influence; and that an electro-positive body repels the fluid from the vicinal part of the body on which it acts, and thus renders it negative ; and the body itself, taken as a whole, becomes electro-polar whether insulated or not.

You may now suppose the polarizing body to be an insulated sphere as represented by p in the opposite figure, and that the nearest body is a brass cylinder, and beyond the cylinder another metallic sphere N is placed, all insulated. Now, accordingly with the doctrine of electro-polarization, the electro-positive sphere p polarizes the cylin

Serjeant Rudd of the Royal Artillery, if still alive, remembers well the effect of an electrical wave. Haring presented his hand to the kite string several times without experiencing even a spark, in the Artillery Barrack grounds at Woolwich, he began to laugh at the idea of electric shocks from

P

N

ITI

der, rendering the vicinal end negative and the remote end positive. The cylinder now beomes a polarizing body, as decidedly as the original electrical sphere P, and in its turn polarizes the sphere x, by repelling its fluid to the remote side.

In all cases of electro-polarization there is a polar axis, which is a line joining those two opposite points of the polar body on which the electric action is exerted to the highest degree. These points are in fact, the real poles of the body. The polar axis of a sphere passes through its centre, and consequently is coincident with a diameter. When the three bodies represented by the figure, have their centres in the same right live, the polarizing axis of the system passes through the centres of all the bodies, as represented by the dotted line.

Now, in order to produce electric waves in the cylinder and the sphere n, we have only to move the polarizing sphere P to and fro, in, or nearly in the axis of polarization represented by the dotted line. Let us suppose, for instance, that the sphere p, is first placed too remote from the cylinder to disturb its natural equilibrium : under these circumstances, both the cylinder and the sphere N will be neutral. Now advance the sphere P gradually towards one end of the cylinder: a corresponding departure of the fluid takes place from the vicinal to the remote parts of the latter, and the gradual accumulation at the remote end produces a corresponding and simultaneous movement of the fluid belonging to the sphere N. In each body, therefore, there is an electric wave during the advance of the disturbing body P. Now withdraw the sphere P: the fluid in both the cylinder and the sphere N flows back again, and the electric waves in those bodies are in the reverse order to the former. It will now be quite obvious that were the sphere P moved to and fro amongst a promiscuous group of bodies, that electric waves would be produced in the group corresponding with the motions of the disturbing electrical sphere.

Hitherto we have considered the bodies to be insulated, and their centres all in the same right line ; but we have seen in a former lecture that if the sphere N were uninsulated the polarization would take place with greater facility and to a greater extent ; and if instead of moving the polarising sphere p in a line with the centres of the other two bodies, the axis of polarization, and consequently the poles of those bodies, would be very differently situated. If the polarizing body p were to move past or over those other two bodies, the axis of polarization would be continually changing its position. Such also, would be the case were the sphere to move over an uninsulated mass of any conducting matter whatever, as is the case in

the air. Shortly, however, I spied a cloud making its appearance behind the Repository, and on its approach asked the serjeant to try again. He did so, but before he got his hand near to the string & discharge struck it, and sent the sceptic reeling, to the great amusement of his brother non-commis. sioned officers who were present.

nature; for when a highly electrized cloud is floating over a tract of country, it polarizes the land as it passes over it, and produces electric waves within the surface, when of good conducting quality, both in the direction of its path and laterally, on both hands, as decidedly as a vessel under sail produces waves on the surface of the water otherwise perfectly at rest.

But it has already been shown that in the clectro-polarization of conducting bodies, the intervening plates of air are also polarized, and consequently highly charged clouds polarize the atmospheric air all around them, repelling the electric fluid to a great distance, and leaving a vicinal negative space on every side. Now imagine the cloud to move on, the air around its path successively yielding to its polarizing influence, suffers its natural share of electric fluid to flow to remoter parts, and the advancing cloud, thus endowed with power, enforces an atmospherical electric tide, in correspondence with its progress through the air. It is this primary tide-wave in the air that polarizes the ground, and produces a corresponding electric wave over the tract of country above which the cloud floats.

When an electric cloud is driven by the wind towards a high piece of ground whose substance is a bad conductor, that substance resists the electric wave, and will not suffer the fluid from the air to transpierce it : an accumulation then takes place on the face of the hill, which becomes charged, in the manner of charged non-conducting solids generally. The consequence is a reaction against the cloud, upon the principle of electric repulsion. The cloud being now under the influence of two forces, the wind and the electric repulsion, will have its speed retarded, if not arrested altogether. Its future path will depend on the relative power of the two forces, and on the direction lines in which they are exerted. Chatham Lines, which is a portion of the great chalk formation, is remarkable for giving new directions of motion to approaching electric clouds. Shooter's Hill, also, I have known to give electric clouds very different directions to those previously pursued from the force of the wind alone. It is far from being an uncommon circumstance to see electric clouds floating in opposition to a light wind; and very frequently indeed, their motions are oblique to it.

Now, since electric clouds travel with the greatest facility over a country which offers the least resistance to the grand electric tide wave, there can be no wonder at their greater tendency to pass over wet land, rivers, &c., than over dry land, which is a worse conductor, especially when such land is high. When clouds are deflected from the wind's direction, they are certain to be guided by the conducting character of the country below. With respect to Chatham Lines and Shooter's Hill, I have had frequent opportunities of observing clouds deflected by them to the respective neighbouring rivers, the Medway and the Thames. From these facts there seems to be a possibility, at least, of forming an idea of the character of the geological strata by observing the motions of thunder clouds and others highly charged with electricity.

That flashes of lightning produce electric waves on every side, may easily be understood by considering what would happen to the cylinder, were the sphere p to be suddenly charged and only for a moment. A sudden and momentary polarization would be the consequence, by an electric wave from the vicinal to the remote end of the cylinder, which would immediately retire again on the exit of the disturbing force from the sphere p. Electric waves produced by flashes of lighting, are necessarily rapid, and only of momentary existence, whilst those produced by clouds are slow and of long continuance. Both classes of electric waves may be illustrated by the following experiments.

Place three or four gold leaf electroscopes in a row, at some distance from one another, on the table, then take hold of the coating of a highly charged Leyden jar, and pass its ball slowly over the electroscopes : the wave thus produced in the air causes their leaves to diverge as this artificial cloud passes over them in succession. If, instead of the usual ball of the Leyden jar, a large well-polished ball were attached to it by a long metal stem, the effect on the electroscopes would indicate the power of this class of waves in a very beautiful manner.

To illustrate electric waves produced by lightning, I place a gold leaf electroscope at a considerable distance from the prime conductor of the machine, and, before the latter becomes electrized, I hold a large brass ball against it. Now turn the machine : nothing happens to the electroscope, but the moment I remove the ball it receives a spark; in fact, a miniature flash of lightning ; and the gold leaves of the electroscope are thrown open, indicating the influence of the momentary wave. In neither case is there left any trace of electric action in the electroscopes. But if each electroscope were to be furnished with a pointed wire, projecting upwards, the whole would remain electrical : the series of electroscopes over which the electric ball passed would show that tall rods pointing into the air receive electric fluid whilst an electric cloud passes over them; and the electroscope charged by the wave from a spark is a good illustration of the electrization of tall pointed rods by a wave from a flash of lightning

Although I have employed the ball of a Leyden jar in these experiments, I am far from supposing that a charged jar is a just resemblance of an electric cloud ; nor do I entertain the idea that the air becomes charged in the manner that coated glass is charged. In my opinion, there is not at the present day a more palpable, certainly not a more popular error, amongst writers on electricity, than that of supposing the air to be charged like glass. About the year 1755, an experiment was established by Æpinus and Wilche, at Berlin, which was supposed to show the identity of charged glass and charged plates of air ; and although the apparatus was neither more nor less

than a condenser on a large scale, the identity was supposed to be proved by means of it; and from that time to the present the same opinion has prevailed, even amongst the most famous of electricians. This Berlinean experiment is, however, exceedingly interesting, from its affording a better exemplification of lightning than any other.

The apparatus for this experiment consists merely of two circular boards, of four or more feet diameter, both of which are covered with tinfoil. When used, one of the boards is suspended by three silken cords to the ceiling, and connected by a wire with the prime conductor. The other is placed, uninsulated, directly beneath the former, their planes being parallel to each other. The adjoining figure will represent

their relative situations. When the machine is put into motion the upper board necessarily becomes electro-positive, and upon the principles of polarization, the lower board becomes electro-negative : and the apparatus is now in the capacity of a condenser. If the action of the machine be now arrested for a moment no charge is to be found in the intervening plate of air, nor any spark discoverable from the charged board, beyond such as is usually found at the prime conductor after the machine has ceased working, but coated glass would have retained the charge discoverable by these means!!!

If now, we place a well polished metallic hemisphere on the middle of the lower board, a series of sparks will strike it from the upper one ; and when the distance is such that the sparks are not frequent, each spark is highly imitative of a flash of lightning, and the noise, though feeble, is the thunder accompanying the miniature flash; for lightning is an electric discharge in the atmosphere, sometimes between a cloud and the earth, but more frequently amongst the clouds themselves. The electric fluid thus discharged flies swiftly through the air, in which it leaves a vacuous track behind; but of momentary duration only: for the displaced air suddenly collapses, and the noise of thunder, as one sudden report, is produced; which, by reverberation amongst the clouds and neighbouring hills, is echoed and re-echoed many times over in a succession of peculiar sounds, of a gradually decreasing intensity, until the last murmur terminates the electro-acoustic event.

The cause of lightning clouds has long been a topic of speculation amongst philosophers. The celebrated Volta, of Como, showed that the vapour of water from the surface of the earth takes up an immense quantity of the electric fluid, and consequently charges the air with it. To illustrate the fact of vapour or steam taking up electric fluid, I have only to place a small tin dish containing water on the cap of the gold-leaf electroscope, and then put a red-hot cinder into the water. This produces a copious evaporation, and

« ZurückWeiter »