consisting of zinc, copper, acid; zinc, copper, acid, there is constituted what is termed a compound galvanic circle. It is by this method that large quantities of electricity are obtained, and which then becomes a highly important chemical ngent, and by which experiments of great brilliancy and interest are performed.

969. The pile of Volta was one of the earliest means by which a compound galvanic series was exhibited. This consisted of a great number of silver or copper coins, and thin pieces of zinc of the same dimensions, together with circular pieces of card, wet with an acid, piled, one series above the other, in the manner shown by fig. 237.

970. The student should be informed that it makes no difference what the metals are which form the galvanic series, provided one be more easily oxidated, or dissolved in an acid, than the other, and that the most oxidable one always forms z the positive side. Thus, copper is s negative when placed with zinc, but becomes positive with silver.

971. The three substances com-, posing the pile, zinc, silver, wet card, and marked Z, S, W, succeed each other in the same order throughout the series, and its power is equal to a single circle, multiplied by the number of times the series is repeated.

TROUGH BATTERY.

972. The galvanic pile is readily constructed, and answers for small experiments, but when large quantities of electricity are required, other means are resorted to, and among these, what is termed the trough battery is the most convenient and efficacious.

973. The zinc and copper plates are fastened to a slip of mahogany wood, and are united in pairs by a piece of metal soldered to each. Each pair is so placed as to enclose a partition of the trough between them, each cell containing a plate of zinc connected with the copper plate of the succeeding cell, and a plate of copper joined with the zinc plate of the preceding cell.

How is the pile of Volta constructed? What qualities are requisite in the two metals in order to yield the galvanic influence? Describe the trough battery.

974. This arrange

inent will be understood by figure 238, where the plates P are connected in the order described, and below them the trough T, to contain the acid into which the plates are to be plunged. 975. The trough is made of wood, with partitions of glass, or what is better, of Wedgewood's ware.

Each trough contains eight or ten cells, which being filled with diluted acid, the plates are suspended and let down into them by means of a pulley. The advantage of this method s, that the plates can be elevated at any moment, and are easily kept clean from rust, without which the galvanic action becomes feeble.

976. A great variety of other forms of metallic combinations have been devised to exhibit the galvanic action, but the same elements, namely, two.metals and an acid, are required in all, nor do the results differ from those above described. The several kinds of galvanic machines already described, are therefore considered sufficient for the objects of this epitome.

ELECTRO-MAGNETISM.

977. Long before the discovery of galvanism, it was sus pected by those who had made the subjects of magnetism and electricity objects of experiment and research, that there existed an affinity or connection between them. In the year 1774, one of the philosophical societies of Germany proposed as the subject of a prize dissertation, the question, "Is there a real and physical analogy between the electric and magnetic forces ?" The question was, however, then answered in the negative; but naturalists still appear to have kept the same subject in view, and by the observation of

What are the advantages of the trough battery? What is said of the suspicion of analogy between electricity and magnetism before the discovery of galvanism?

new facts, the existence of such an analogy was from time to time affirmed by various philosophers.

978. The aurora borealis, which has long been supposed to be an electrical phenomenon, was observed to influence the magnetic needle; and lightning, well known to be nothing more than an electrical movement, was known in many instances to have destroyed or reversed the polarity of the compass.

979. An instance of this kind, which might have led to very disastrous consequences, is related of a ship in the midst of the Atlantic, which being struck with lightning, had the polarity of all her compasses reversed. This being unknown, the ship was directed as usual by the compass, until the ensuing evening, when the stars showed that her direction was in the exact opposite course from what was intended, and then it was that the phenomenon in question was first suspected.

980. These discoveries of course led philosophers to try the effects of powerful electrical batteries on pieces of steel, and although polarity was often induced in this manner, yet the results were far from being uniform, and the experiments on this subject seem in a measure to have ceased, when the discovery of the galvanic influence opened a new field of inquiry, and gave such an impulse to the labours, investigations, and experiments of philosophers throughout Europe, as perhaps no other subject had ever done.

981. It was, however, more than twenty years from the time of Galvani's discovery, before the science of ElectroMagnetism was developed, the first having taken place in 1791, while the experiments of M. Oersted, the real discoverer of Electro-Magnetism, were made in 1819.

982. M. Oersted was Professor of Natural Philosophy, and Secretary to the Royal Society of Copenhagen. His experiments, and others on the subject in question, are detailed at considerable length, and illustrated by many draw. ings, but we shall here only give such an abstract as to make the subject clearly understood.

983. The two poles of the battery, fig. 255, are connected by means of a copper wire of a yard or two in length, the two parts being supported on a table in a north and south direction, for some of the experiments, but in others the di

Is there any connection between the aurora borealis and the magnetic needle? What is said to have been the effect of lightning on the compasses of a ship at sea? What is the uniting wire?

rection must be changed, as will be seen. be remembered, is called the uniting wire.

This wire, it will

984. Being thus prepared, and the galvanic battery in action, take a magnetic needle six or eight inches long, properly balanced on its pivot, and having detached the wire from one of the poles, place the magnetic needle under the wire, but parallel with it, and having waited a moment for the vibrations to cease, attach the uniting wire to the pole. The instant this is done, and the galvanic circuit completed, the needle will deviate from its north and south position, turning towards the east or west, according to the direction in which the galvanic current flows. If the current flows from the north, or the end of the wire along which it passes to the south is connected with the positive side of the battery, then the north pole of the needle will turn towards the east; but if the direction of the current is changed, the same pole will turn in the opposite direction.

985. If the uniting wire is placed under the needle, instead of over it, as in the above experiment, the contrary ef fect will be produced, and the north pole will deviate to wards the west.

N'

Fig. 239.

E

986. These deviations will be understood by the follow ing figures. In fig. 239, N presents the north, and the south pole of the magnetic needle, and p the positive and n the P negative ends of the uniting wire. The galvanic current, therefore, flows from p towards N n, or, the wire being parallel with the needle, from the north towards the south, as shown by the direction of the arrow in the figure.

W

Now the uniting wire being above the needle, the polt N, which is towards the positive side of the battery, will deviate towards the east, and the needle will assume the direc tion N' S'.

On the contrary, when the uniting wire is carried below the needle, the galvanic current being in the same direction as before, as shown by fig. 240, then the same, or north pole, will deviate towards the west, or in the contrary direction from the former, and the needle will assume the position N' S'.

If the needle is stationary, and the current flows from the north, what way will the needle turn? Explain fig. 239.