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Calorific Effects of Electricity.

439

liquids, as ether or alcohol, it inflames them. An arrangement for effecting this is represented in fig. 343. It is a small glass cup through the bottom of which passes a metal rod, terminating in a knob and fixed to a metal foot. A quantity of liquid sufficient to cover the knob is placed in the vessel. The outer coating of the jar having been connected with the foot by means of a chain, the spark which passes when the two knobs are brought near each

Fig. 342.

n

m

Fig. 343.

a

other, inflames the liquid. With ether the experiment succeeds very well, but alcohol requires to be first warmed.

Coal gas may also be ignited by means of the electric spark. A person standing on an insulated stool places one hand on the conductor of a machine which is then worked, while he presents the other to the jet of gas issuing from a metallic burner. The spark which passes ignites the gas. This experiment may be curiously varied by igniting the gas by means of a piece of ice held in the hand.

When a battery is discharged through a metal wire it becomes incandescent, and may be melted or even volatilised provided the charge be sufficiently powerful.

For this experiment an apparatus is used which is called the universal discharger, for it may be employed in a host of experiments on the electrical discharge. It consists (fig. 344) of two brass rods, A and B, each insulated on a glass stem. These rods can slide along hinged joints, so that they can be placed at any distance from each other and inclined in any direction. Between

them is a small table support, which can be placed at any height, and which is intended to support objects which are to be submitted to the action of the discharge.

To melt a metal wire it is fixed at i to two knobs fastened on the rods, then connecting one of these by means of a chain with the outside of a powerful battery, the other is brought in contact with the inner coating, either by means of the discharging rod, or by a chain attached to a metal rod fixed on a glass handle. The moment the spark passes between the knob and the battery, the

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Electrical Portraits.

441

wire of it is fine enough, is melted in incandescent globules, and is even volatilised, that is, converted into vapours which disappear in the atmosphere. If the wire is thicker it simply becomes red hot but does not melt, and if still larger it is merely heated without becoming luminous.

When an electric discharge is sent through gunpowder placed on the table of a Henley's discharger, it is not ignited, but is projected in all directions. But if a wet string be interposed in the circuit, a spark passes which ignites the powder. This arises from the retardation which electricity experiences in traversing a semi-conductor, such as a wet string; for the heating effect is proportional to the duration of the discharge.

422. Electrical portraits.-The fusion of metals by the electrical discharge is applied to make what are called electrical portraits.

For this purpose a thin card is taken of the shape abm, and the design to be copied is cut out; a sheet of tinfoil is fastened on the rest of the card at a and b, but not at c. A leaf of gold is then placed upon the design, care being taken that it touches both the pieces of tinfoil, a and b. The lateral portion of the card, m, is then bent over, the card placed on a silk ribbon, and the whole pressed in a frame, P. When the discharge is passed from a to b, the tinfoil being thicker is not melted; but the gold which is very thin is volatilised, and forms on the ribbon through the pattern a brown coating, which reproduces all the details as seen in R.

423. Mechanical effects.-The mechanical effects are the violent lacerations, fractures, and sudden expansions which ensue when a powerful discharge is passed through a badly conducting substance. Glass is perforated, wood and stones are fractured, and gases and liquids are violently disturbed. The mechanical effects of elec

tricity may be demonstrated by a variety of experiments. The body to be submitted to experiment is placed on the plate, N, in contact with the two knobs which terminate the rods, A and B, so that they cannot receive the discharge without transmitting it to the object on the table. Thus, for instance, if a piece of wood is placed so as to be struck in the direction of the fibres, it is smashed into pieces the moment the discharge passes.

Fig. 336 represents an arrangement for perforating a piece of glass or card. It consists of two glass columns, with a horizontal cross piece, in which is a pointed conductor, B. The piece of

Fig. 346.

glass, A, is placed on an insulating glass support, in which is placed a second conductor, terminating also in a point, which is connected with the outside of the battery, while the knob of the inner coating is brought near the knob of B. When the discharge passes between the two conductors the glass is perforated. The experiment only succeeds with a single jar when the glass is very thin; otherwise a battery must be used.

424. Chemical effects.-The chemical effects are the decompositions and recombinations effected by the passage of the elec

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Atmospheric Electricity.

443

trical discharge. When two gases which act on each other are mixed in the proportions in which they combine, a single spark is generally necessary to determine their combination; but, where either of them is in great excess, a succession of sparks is necessary. Priestley found, that when a series of electric sparks was passed through moist air, its volume diminished, and blue litmus introduced into the vessel was reddened. This, Cavendish found, was due to the formation of nitric acid.

Among the chemical effects must be enumerated the formation of ozone, which is recognised by its peculiar odour and by certain chemical properties. The odour is perceived when electricity issues through a series of points from a conductor into the air. Its true nature is not accurately known: some regard it, and with great probability, as an allotropic modification of oxygen, and others as a teroxide of hydrogen.

425. Magnetic effects.-By the discharge of a large Leyden jar or battery, a steel wire may be magnetised if it is laid at right angles to the conducting wire, through which the discharge is passed, either in contact with the wire or at some slight distance. And even with less powerful discharges a steel bar or needle may be magnetised by placing it in a tube on which is coiled a fine insulated copper wire. On passing the discharge through this wire the steel becomes magnetised.

CHAPTER VI.

ATMOSPHERIC ELECTRICITY. THUNDER AND LIGHTNING.

426. Identity of thunder and lightning.—The first physicists who observed the zigzag motion of the electric spark, compared it to the gleam of lightning, and its crackling to the sound of thunder. But Franklin, by the aid of powerful electrical batteries, first established a complete parallel between lightning and electricity; and he indicated, in a memoir published in 1749, the experiments necessary to attract electricity from the clouds by means of pointed rods. The electric fluid, said he, in concluding his memoir, is attracted by points; we know not whether lightning is endowed with the same property; but, since electricity and lightning agree in all other respects, it is probable they will not differ in this; and the experiment