710 The Mechanical, Heating, and Luminous bc (fig. 262), hinged together like the legs of a pair of tongs, with knobs at the ends, and with a glass handle, d. If one knob be a Fig. 262. placed in contact with a conductor which communicates with either the external or internal surface of a battery, and if the other knob is then made to touch some conducting substance which leads to the other surface, the discharge instantly takes place, and electrical equilibrium is restored. It remains to enumerate some of the many experiments, both instructive and interesting, which may be performed with the apparatus we have described. These are, however, so numerous, that in a work like the present we shall attempt to give only typical experiments, leaving fuller catalogues and minutiæ to special treatises on this subject, which, within so very short space of time, has grown from infantile to giant proportions. 949. In the first place, the mechanical effects of attraction and repulsion are exhibited by the following experi ments : A common glass tumbler, strongly electrified by the prime conductor of a machine, and inverted over a few pith balls on a table, causes an amusing dance of the balls between the tumbler and the table. The dance will continue till they carry all the electricity of the tumbler to the earth. Pith figures may be made to dance by the unseen power of electricity between a metal plate, hung by a chain from the prime conductor of an electric machine, and a similar plate placed beneath, in connection with the earth. A small paper kite, tied by a string to the prime conductor of a machine, will get electrified and then keep floating, by electric repulsion, round the knob. If a few pieces of paper, or pith balls, or wafers, be laid on the lid of an electrophorus, these will, on our lifting the cover, get electrified and repelled, floating away in a curious manner. If an excited stick of sealing-wax be brought near to a fine jet of water, the electrification of the water by induction causes the jet to keep together longer than it otherwise would. A Leyden jar discharged against a plate of resin or vulcanite over which some fine lycopodium powder is strewed, causes a curious disposition of the powder, which is very different in type accor.ling Effects of the Electric Discharge. 711 as the charge communicated is positive or negative. These are known as Lichtenberg's Figures. If a person stand on an insulating stool, such as a stool with glass legs, with one hand on the prime conductor of a machine in good working order, he becomes electrified, and the hairs of his head stand on end in consequence of their mutual repulsion. A person standing near, may draw sparks from his electrified neighbour; and the latter, by simply putting his finger or knuckle to a gas-jet, is able to inflame it. If a wire ending in a fine point be fixed on the prime conductor, the electrification and consequent repulsion of the air-particles by the point is felt on the hand or face like wind blowing from the point. This electric wind will suffice to drive a small wheel or orrery with paper vanes. If a small tin pail have holes pierced in its bottom, so fine that vater poured into it will issue only drop by drop, the electrification of the water will cause it to issue in fine streamlets. 950. Secondly, the heating and luminous effects of the elec tric discharge may be shown by a variety of pleasing ex- The bright spark seen when the knuckle or a conductor is presented to an electrified body, is due either to the incandescence of minute particles of the conductor, or to combustion of the atmospheric gases. This spark passes into a diffused light when the discharge takes place in rarefied air, as, for instance, between a platinum wire sealed in the close end of a barometer-tube and a mercurial column. Spectroscopic examination of this light shows it to be due to the incandescence of the aërial traces remaining in the tube, and of the vapour of mercury. In a perfect vacuum, however, it is found that no discharge takes place, showing that electricity must always have a material means of conduction. The spectrum of the Aurora borealis shows distinctly the lines due to the incandescence of nitrogen, proving that this phenomenon is due to diffuse electric discharges in the higher and rarefied regions of the atmosphere. When the electric spark is examined in the dark, it exhibits the following peculiarities :-When short and strong, it is a straight stream of fire between the two conductors; when the spark passes over a considerable distance, it is crooked and branching, like the tributaries of a stream; the divergence from the straight path being 712 Velocity of the Electric Discharge. in all probability due to the interference of dust particles flɔating in the air. When the machine is worked in the dark, and in good condition, spontaneous discharges into the air occur; these take the form of a brush ordinarily; and if a conductor be presented to this brush, the streams of light converge upon it; if, however, the conductor be fitted with a fine point, no brush appears; the electricity passes silently into the air, and the point is crowned with a simple spot or glow of light. The light of the electric discharge also exhibits differences of colour, according to the nature of the metals between which it takes place. Holtz's machine is particularly fitted to show these luminous effects of the discharge. If a Leyden jar or battery, strongly charged, be discharged through a leaf of gold, silver, tin, or copper, or a fine iron wire, the heat of the discharge is sufficient to burn up the metals, or even dissipate them into vapour. The spark of a small jar, or even of an electrophorus, is sufficient to inflame a mixture of explosive gases, such as oxygen and hydrogen, or common coal-gas and air, and is employed constantly in chemical laboratories for this purpose. Toy pistols and toy cannon are constructed on this principle; they are filled with a mixture of common coal-gas and air, and the mouth plugged air-tight with a cork or wooden ball. The passage of a spark between two metal balls inside suffices to explode the mixture and project the ball. If a strongly-charged Leyden jar be discharged amongst some dry gunpowder, the duration of the spark is too short to inflame the powder; but if a bad conductor, such as a wet string, be introduced in the passage of the electricity betweeen the outside and inside coatings of the jar, the powder will be readily fired. If the discharge be passed through a glass tube filled with water, by plugging the ends of the tube with corks and passing in copper wires till they are within a quarter of an inch or so, the mechanical shock due to the sudden electrification and mutual repulsion of the water particles is sufficient to shiver the tube and scatter its contents. 951. The intensity and suddenness of the electric spark and light is well illustrated by causing a variously-coloured disc to be rotated very quickly. Of course in ordinary light the impression is a fusion of all the different colours; but if a strong spark from the jar or battery be passed near the face of the disc while rapidly revolving, so as to illuminate it for an instant, the individual colours start forth in surprising distinctness, just as if the disc were brought for a Velocity of Lightning. 713 moment to perfect rest. The late Sir Charles Wheatstone found the duration of the spark to be 24000th part of a second for a maximum duration, and in some cases less than the millionth of a second. From a calculation made by M. Arago, a millionth of a second may be taken as the ordinary duration of a flash of lightning. Admitting that this is only an approximation to the truth, it brings before us the extraordinary quickness of vision; for while the flash lasts all objects are visible to the eye, and yet the light whereby they are seen must be reflected from them in an inconceivably short period of time. Thus, if a body in swift motion be illuminated by an electric or lightning flash, it will appear motionless for a moment. A cannon ball would appear to stand still in its course for an instant, and a falling sheet of water would be seen to be really discontinuous by the aid of the electric flash. For a similar reason, in a thunderstorm during the night at sea, the waves, although in violent motion, appear to be suddenly petrified or at perfect rest during the lightning flash; and, under similar circumstances on land, the leaves and branches of the trees, although strongly agitated by the wind, appear to become suddenly still. It may have been observed by those who have travelled in an express railway train at the rate of from sixty to eighty feet in a second, that near objects in a cutting, or on a wall or embankment, appear to lose their form and to fly past us with inconceivable rapidity. We see them, but they become greatly lengthened, owing to the duration of the impression on the eye. A large rounded pebble is no longer seen as such, but it presents the appearance of a brown streak, six or eight feet in length; and all bodies by the side of the road appear to run into parallel lines, just as a stick lighted at one end, when swung round with great velocity, appears to the eye like a circle of fire. Perhaps nothing can more strongly indicate the inconceivable velocity of lightning, than the fact that objects thus seen by the flash during darkness in railway travelling, appear in their proper shape and position, as if the motion of the train had been suddenly and completely destroyed. If a strip of metallic foil, such as tinfoil, adhering to the surface of a glass plate or tube, have divisions or small open gaps cut in it, the interruption of an electric flash becomes visible at every one as a bright spark. Thus luminous letters or writing, consisting of dotted lines of light, are producible at each electric discharge, and many pretty devices can thus be exhibited. 714 Atmospheric Electricity. The electricity of friction is capable of producing also magnetic and chemical effects; but as these are analogous to the effects more powerfully produced by galvanic electricity, they need not be here detailed.* The Electricity of the Atmosphere. 952. By the lightning flash between a cloud and the earth, all the effects of the electric spark mentioned above may be produced in intensified degrees. Dr. Franklin, in the middle of last century, was the first to show the identity of the lightning flash with the electric spark. He sent up a kite into the air in the midst of a thunderstorm, having provided the kite with an iron point connected with the hempen string. To the lower end of the string an iron key was attached, and the latter again attached to a strong silk string, so as to insulate it from the hand of the person holding. After waiting some time he was able to draw an electric spark from the key with his knuckle; and a shower having improved the conducting power of the string, the philosopher was able to charge a Leyden jar with the electricity of the clouds, and so prove its identity with the ordinary excitement of the cylinder machine. The result of this experiment was the devising of pointed metal rods for leading the electricity of the clouds harmlessly to the earth, and thus saving life and property from the destructive force of sudden electric discharges of clouds. The writer refers to this fact with particular interest, from having twice witnessed, during a voyage made in early life, before the adoption of the more substantial lightning-rods now generally supplied to great ships, the appalling occurrence of a ship insufficiently protected being struck by lightning. The first time was in the South Atlantic, where a mast was split, and of several men knocked down one did not recover; the second time, when the ship was at anchor in the Straits of Malacca. A part of the rigging was set fire to, but by prompt measures the ship was saved. A badly constructed lightning conductor would, however, prove *The production of electricity by friction is remarkably exemplified in the escape of steam at high pressure from a steam-engine placed on a nonconducting surface. The friction thus produced generates electricity on a large scale, and thus sparks have been obtained from a locomotive placed on dry bricks, owing to the friction of the steam escaping from the valve. The engine-driver, in attempting to move the lever of a locomotive so placed, received a succession of shocks. |