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Magic Pane.

477

from the recomposition of the positive fluid of the upper cap with the negative of the lower. If the air be gradually allowed to enter by opening the stopcock, the resistance increases, and the light, which appeared continuous, white, and brilliant, is now only seen as an ordinary spark.

433. Magic pane.-The magic pane consists of a glass plate,

one side of which is covered with several strips of tinfoil, arranged so as to form a series of metallic bands, parallel and close to each other. The pane is supported vertically by two glass rods, and the upper end of the tinfoil is connected with the electrical machine by a conductor, and the lower one with the ground by a chain. In this condition, if the machine be worked, the electricity will pass into the ground by the tinfoil, without any inter

ruption; but if a series of breaks are made in the tinfoil by cutting it away with a penknife, a spark appears at each break; and if these breaks be so arranged as to represent a given object, a flower, or a monument, or words, these objects are reproduced in a line of fire when the electrical machine is set to work. This experiment is really due to the prodigious velocity of electricity, which is not less than about 186,000 miles in a second; hence in the above experiment, although the sparks are really successive, they follow each other with such rapidity as to seem continuous (363).

434. Luminous globe and tube.-The luminous globe is a

Fig. 396.

glass globe lined on the inside with a series of small lozenges of tinfoil placed very near each other without actually touching. The first plate is connected with an electrical machine at work, and the last with the ground, upon which a series of bright sparks appears at each break in the metallic conductor (fig. 396).

If the small metal plates are arranged inside a glass tube in the form of a spiral from one end to the other, this arrangement forms a luminous tube.

435. Volta's cannon.-This is not merely interesting as an experiment, but also as demonstrating an important fact, namely, that

-435]

Volta's Cannon.

479 the electrical spark can establish chemical action. Thus, water is formed of two gases, hydrogen and oxygen, in the ratio of one volume of the latter to two volumes of the former. Now when an electrical spark is passed through a mixture of these two gases, they combine in these proportions, and form water. This combination is, moreover, attended by a bright flash of light and a loud report, the latter being due to the expansive force of aqueous vapour, arising from the high temperature produced by the combination.

On this property which mixtures have of detonating by the electrical spark, Volta's cannon, represented in fig. 396, is constructed. It is a small brass cannon resting on an insulating support. In the touchhole is a small glass tube, and in this a brass wire with a small knob at each end; one of which knobs is on the outside, and the other very near the inside of the cannon but not touching it. Having introduced a mixture of two parts of hydrogen and one of oxygen, the cannon is closed by a cork and is connected with the ground by a metal chain. If then the charged disc of the electrophorus be approached, a spark passes to the small knob, and at the same time inside the cannon. This latter causes the two gases to combine with a violent explosion, which drives out the cork.

CHAPTER IV.

CONDENSATION OF ELECTRICITY.

436. Discovery of the Leyden jar.—In 1745 Cuneus of Leyden, wishing to electrify water contained in a flask, suspended to the conductor of an electrical machine a wire, and then held the flask

Fig. 397.

in one hand so that the wire just dipped in the water (fig. 397). The machine having been worked for some time he accidentally touched the conductor, and in so doing received a violent shock. Muschenbroeck, his teacher, who repeated the experiment, received in the arms and breast a shock so violent that it was two days before he recovered from its effects; and, writing to his friend Réaumur, he said he would not repeat the experiment for the whole kingdom of France.

In the previous year Kleist, a German clergyman, in a private

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Electrical Condensers.

481

letter to a friend, described an experiment which he had made, and which was substantially the same as the above; but it was the Dutch philosophers who investigated the conditions of success, and who the explanation of the phenomenon ; and accordingly it is in their honour that the name 'Leyden jar' is given to the apparatus to which their discovery gave rise.

gave

437. Electrical condensers. It is not difficult to see that in the above experiments the water and the hand play the part of two conductors separated by an insulating plate; any arrangement in which these conditions are fulfilled would produce similar effects; for it would have the power of accumulating or condensing electricity, from which has been derived the term accumulator or condenser.

The action of the condenser may be most conveniently explained by reference to that of Epinus, which consists of two metal plates, A and B, insulated by being supported on glass legs (fig. 398); between them is a pane of ordinary glass, of somewhat larger diameter than that of the plates A and B, which are about six inches in diameter. The legs can be moved along a support, and fixed in any position.

In explaining the action of the condenser, it will be convenient to call that side the metal plate nearest the glass the anterior, and the other the posterior, side. And first let A be at such a distance

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