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Ques. In cases where part of the electricity is transferred from the body originally electrified to the body touched what is this transfer of electricity called?

Ans. It is called a discharge.

Ques. When does a discharge of electricity become a

current?

Ans. When electricity is supplied to the body as fast as it is taken away.

In considering the practical questions relating to the electric current three characteristic effects need to be thought of: The chemical.

I.

All lines of magnectic force from closed circuits and an electric circuit, so called, may be said to be the path in which electricity passes from a given point around a conducting point back again to its starting point.

The discovery of electric currents originated with Galvani, a physician of Bologna, who about the year 1786, made a series of curious and important observations upon the convulsive motions produced by the "return shock" and other electric discharges upon a frog's leg.

This immortal discovery arose in the most immediate and direct manner, from an indisposition with which a Bolognese lady was affected, for which her medical adviser prescribed frog-broth.

Galvani, the husband of the lady, was Professor of Anatomy in the University of Bologna. It happened that several frogs,

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prepared for cooking, lay upon the table of his laboratory, near to which his assistant was occupied with an electrical machine. On taking sparks from time to time from the conductor, the limbs of the frogs were affeeted with convulsive movements resembling vital actions.

This was the effect of the inductive action of the electricity of the conductor upon the highly electroscopic organs of the frogs; but Galvani was not sufficiently conversant with this branch of physics to comprehend it, and consequently regarded it as a new phenomenon. He proceeded to submit the limbs of frogs to a course of experiments, with the view to ascertain the cause of what appeared to him so strange. For this purpose, he dissected several frogs, separating the legs, thighs, and lower part of the spinal column from the remainder, so as to lay bare the lumbar nerves. He then passed copper hooks through that part of the dorsal column which remained above the junction of the thighs, without any scientific object, but merely for the convenience of suspending them until required for experiment. It chanced, also, that he suspended these copper hooks upon the iron bar of the balcony of his window, when. to his inexpressible astonishment, he found that whenever the wind or any other accidental cause brought the muscles of the leg into contact with the iron bar, that a similar convulsive kick was produced in the frog's leg.

Galvani imagined this action to be due to electricity generated by the frog's leg itself. It was, however, proved by Volta, Professor in the University of Pavia, that the electricity arose not from the muscle or nerve, but from the contact of

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the dissimilar metals. A greater accumulation of electric energy at one point than at another is what causes the electric current. Such a current always flows through a conducting body if the ends are kept at a different electrical pressure.

In order that a continuous flow may be kept up there must be a circuit provided. Currents are called continuous if they flow without stopping in one direction. They are called alternate currents if they continually reverse in direction in a regular periodic manner, flowing first in one direction round the circuit and then in the other.

The flow would cease unless the difference in electric pressure is maintained; this can be done by either a battery or dynamo. The current is impelled through the circuit by the electric pressure at the battery or dynamo just as a current of steam is impelled through pipes, of whatever form, by the pressure of the steam formed in the boiler.

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All dynamos, of whatever form, are based upon the discovery made by Faraday in 1831, that electric currents are generated in conductors by moving them in a magnetic field. Faraday's principle may be enunciated as follows : When a conductor is moved in a field of magnetic force in any way so as to cut the lines of force, there is an electromotive-force produced in the conductor, in a direction at right angles to the direction of the motion, and at right angles also to the direction of the lines of force, and to the right of the lines of force, as viewed from the point from which the motion originates.”

To understand clearly Faraday's principle-that is to say, how is it that the act of moving a wire so as to cut magnetic

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lines of force can generate a current of electricity in that wire -let us repeat and further explain this.

A wire through which a current of electricity is flowing looks in no way different from any other wire. No man has ever yet seen the electricity running along in a wire, or knows

precisely what is happening there, but no electrician is in any doubt as to one most vital matter, namely, that when that which is called an electric current flows through a wire, the magnetic forces with which that wire is thereby, for the time, endowed, resides not in the wire at all, but in the space sur rounding it. Every one knows that the space or "field" surrounding a magnet is full of magnetic "lines of force,"

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and that these lines run in tufts from the N-pointing pole to the S-pointing pole of the magnet, invisible until, by dusting iron filings into the field, their presence is made known, though they are always in reality there (Fig. 27). A view of the magnetic field at the pole of a bar magnet, as seen end-on, would, of course, exhibit merely radial lines, as seen in Fig. 28.

Now, every electric current (so-called) is surrounded by a magnetic field, the lines of which can be similarly revealed. To observe them, a hole is bored through a card or a piece of glass, and the wire which carries the current must be passed up through the hole. When iron filings are dusted into the field they assume the form of concentric circles (Fig. 12),