-459]

Aurora Borealis.

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459. Aurora borealis.-The aurora borealis, or northern light, or more properly polar aurora, is a remarkable luminous phenomenon which is frequently seen in the atmosphere at the two terrestrial poles, but more especially at the north pole (fig. 419). At the close of the day an indistinct light appears in the horizon in the direction of the magnetic meridian. This luminosity gradually changes into a regular arc of a pale yellow with its concave side turned towards the earth. Finally, the rays burst all over the horizon, passing necessarily from yellow to deep green,

and to the most brilliant purple. All these rays converge towards one point of the horizon, which is in the prolongation of the line of the dipping needle, and they form then a fragment of an immense luminous cupola.

When the luminous arc is formed it often remains visible for some hours; then the lustre diminishes, the colours disappear, and this brilliant phenomenon gradually diminishes, or is suddenly extinguished.

Numerous hypotheses have been devised to account for the aurora boreales. The constant direction of their arc as regards the magnetic meridian, and their action on the magnetic needle (405),

suggest that they ought to be attributed to electric currents in the higher regions of the atmosphere. This hypothesis is confirmed by the circumstance that during the prevalence of the aurora borealis, electric telegraph lines are spontaneously affected in a powerful but irregular manner; needles are deflected, armatures attracted, and alarums rung. This interference is at times so serious, especially in northern countries, that it is necessary to suspend the ordinary transmission of telegraphic messages.

According to De la Rive, the aurora boreales are due to electric discharges which take place in polar regions between the positive electricity of the atmosphere and the negative electricity of the terrestrial globe; electricities which themselves are separated by the action of the sun, principally in the equatorial regions.

In Chapter XII. an experiment will be described which De la Rive devised in support of this hypothesis.

460. St. Elmo's fire. This name is given by sailors to the luminous brushes or stars which sometimes appear at the tops of masts and yards of vessels, and which are often accompanied by a crackling sound, resembling that heard when sparks are taken from electrical machines.

These luminous effects were known to the ancients. Pliny speaks of the fiery stars seen on the ends of soldiers' lances. When they were two in number they were compared to Castor and Pollux, and that was a favourable presage; if only one appeared, it was likened to their sister Helena, which was considered a bad omen.

St. Elmo's fire is a simple case of induction. The atmospheric electricity acting on conductors decomposes the neutral fluid, attracting the contrary electricity, which, from the power of points, being liberated at the extremities of the masts, or by the metal of the lances, gives rise to the luminous brush. The same effect is observed when, placing a metal point on the conductor of the electrical machine, it is made to work in darkness.

461. Atmospheric electricity on the Pyramids.—Some curious observations were made by Siemens on the summit of the Cheops pyramid during the prevalence of the kamsin (297). On holding his finger out a peculiar hissing sound was heard, and at the same time a prickly sensation was felt. On holding in one hand a filled champagne bottle, the cork of which was coated with tinfoil, the same sound was heard, and sparks continually passed from the label to the hand which held the flask, and when Siemens touched the top with the other hand he experienced a powerful shock.

-461] Atmospheric Electricity on the Pyramids.

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In this case the liquid which was in conducting communication with the cork formed the inner coating of a Leyden jar, the outer coating of which was formed by the label and by the hand.

When the jar was improved by coating it with moistened paper it gave such powerful discharges, with a striking distance of half an inch, that an Arab who held Siemens' hand was thrown to the ground as if struck by lightning, when Siemens presented the bottle to his nose.

CHAPTER VII.

ELECTRICITY DUE TO CHEMICAL ACTION.

VOLTAIC BATTERY.

cerned with the latter.

462. Galvani's experiment.—We have already seen that the two most powerful sources of electricity are friction and chemical combination. Having described the former, we are now to be conYet it may be premised that this is not a new kind of electricity, but only another method for its production far more abundant than friction, and leading to the most remarkable effects.

To Galvani, professor of anatomy in Bologna, is due the discovery in 1790 of these new electrical phenomena, to which he was led by a casual observation. It is said that a dead frog was accidentally suspended by a hook of copper to the iron railings of a balcony; it was observed to be violently contracted whenever the legs of the animal came in contact with the iron bars. Galvani's observation may be reproduced in the following manner: the legs of a recently killed frog are prepared, and suspended to a copper hook, which passes between the vertebral column and the nerve filaments on each side of it. If then the copper support and the legs are momentarily connected by a plate of zinc, a smart contraction of the muscles ensues at each contact (fig. 420). Galvani had some time before observed that the electricity of machines produced in dead frogs analogous contractions, and

Fig. 420.

464]

Voltaic Pile.

511 he attributed the phenomena first described to an electricity inherent in the animal. He assumed that this electricity, which he called vital fluid, passed from the nerves to the muscles by the metallic arc, and was thus the cause of contraction. This theory met with great support, especially among physiologists, but it was not without opponents. The most considerable of these was Alexander Volta, professor of physics in Pavia.

463. Volta's fundamental experiment.-Galvani's attention had been exclusively devoted to the nerves and muscles of the frog ; Volta's was directed upon the connecting metal. Resting on the observation, which Galvani had also made, that the contraction is more energetic when the connecting arc is composed of two metals, than when there is only one, Volta attributed to the metals the active part in the phenomenon of contraction. He assumed that the disengagement of electricity was due to their contact, and that the animal parts only officiated as conductors, and at the same time as a very sensitive electroscope.

By means of the then recently invented electroscope, Volta devised several modes of showing the disengagement of electricity on the contact of metals, of which the following is the easiest to perform :

The moistened finger being placed on the upper plate of a condensing electroscope (fig. 406), the lower plate is touched with a plate of copper, c, soldered to a plate of zinc, z, which is held in the other hand. On breaking the connection and lifting the upper plate (fig. 407), the gold leaves diverge, and, as may be proved, with negative electricity. Hence, when soldered together, the copper is charged with negative electricity, and the zinc with positive electricity. The electricity could not be due either to friction or pressure; for if the condenser plate, which is of copper, is touched with the zinc plate, z, the copper plate to which it is soldered being held in the hand, no trace of electricity is observed.

A memorable controversy arose between Galvani and Volta. The latter was led to give greater extension to his contact theory, and propounded the principle that when two heterogeneous substances are placed in contact, one of them always assumes the positive and the other the negative electrical condition. In this form Volta's theory obtained the assent of the principal philosophers of his time.

464. Voltaic pile.-Reasoning from this theory of contact, Volta was led in 1800 to the invention of the marvellous instrument which