CHAPTER XV

INDUCED CURRENTS

THE PRINCIPLE OF THE DYNAMO AND MOTOR

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346. Current induced by a magnet. Let 400 or 500 turns of No. 22 copper wire be wound into a coil C (Fig. 299) about two and a half inches in diameter. Let this coil be connected into circuit with a lecture-table galvanometer (Fig. 263), or even a simple detector made by suspending in a box, with No. 40 copper wire, a coil of 200 turns of No. 30 copper wire (see Fig. 299). Let the coil C be thrust suddenly over the N pole of a strong horseshoe magnet. The deflection of the pointer p of the galvanometer will indicate a momentary current flowing through the coil. Let the coil be held stationary over the magnet. The pointer will be found to come to rest in its natural position. Now let the coil be removed suddenly from the pole. The pointer will move in a direction opposite to that of its first deflection, showing that a reverse current is now being generated in the coil.

FIG. 299. Induction of electric currents by magnets

We learn, therefore, that a current of electricity may be induced in a conductor by causing the latter to move through a magnetic field, while a magnet has no such influence upon a conductor which is at rest with respect to the field. This discovery, one of the most important in the history of science, was announced by the great Faraday in 1831. From it have sprung directly most of the modern industrial developments of electricity.

Famous English physicist and chemist; one of the most gifted of experimenters; son of a poor blacksmith; apprenticed at the age of thirteen to a London bookbinder, with whom he worked nine years; applied for a position in Sir Humphry Davy's laboratory at the Royal Institution in 1813; became director of this laboratory in 1825; discovered electromagnetic induction in 1831; made the first dynamo; discovered in 1833 the laws of electrolysis, now known as Faraday's laws; the farad, the practical unit of electrical capacity, is named in his honor

One of the most familiar of the more recent applications of the great principle of induction discovered by Faraday is the induction motor, which has come into extensive use in both large and small sizes. The particular one here shown is known as the squirrel-cage form, in which there is no electrical connection between the stator (the stationary part) and the rotor (the revolving part). The stator is wound on a laminated core like the stator of a dynamo, while the rotor consists of copper bars laid in a slotted laminated core, their ends being joined to copper rings, one at each end. The bars are therefore in parallel. The alternating current applied to the stator windings develops a magnetic field which rotates around the iron ring of the stator. This is equivalent to a set of magnetic poles mechanically rotated around the rotor. The magnetic lines of force which therefore cut across the copper bars of the rotor generate in them an E.M.F. which causes a current to flow in the copper system of the rotor. The rotating field reacts with the field produced by the current in the conductors of the rotor so as to cause the rotor to be dragged around after the rotating field

347. Direction of induced current. Lenz's law. In order to find the direction of the induced current, let a very small P.D. from a galvanic cell be applied to the terminals A and B (Fig. 299), and note the direction in which the pointer moves when the current enters, say, at A. This will at once show in what direction the current was flowing in the coil C' when it was being thrust over the N pole. By a simple application to C of the right-hand rule (§ 308) we can then tell which was the N and which the S face of the coil when the induced current was flowing through it. In this way it will be found that if the coil was being moved past the N pole of the magnet, the current induced in it was in such a direction as to make the lower face of the coil an N pole during the downward motion and an S pole during the upward motion. In the first case the repulsion of the N pole of the magnet and the N pole of the coil tended to oppose the motion of the coil while it was moving from a to b, and the attraction of the N pole of the magnet and the S pole of the coil tended to oppose the motion while it was moving from b to c. In the second case the repulsion of the two N poles tended to oppose the motion between b and c, and the attraction between the N pole of the magnet and the S pole of the coil tended to oppose the upward motion from b to a. In every case, therefore, the motion is made against an opposing force.

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From these experiments, and others like them, we arrive at the following law: Whenever a current is induced by the relative motion of a magnetic field and a conductor, the direction of the induced current is always such as to set up a magnetic field which opposes the motion. This is Lenz's law. This law might have been predicted at once from the principle of the conservation of energy; for this principle tells us that since an electric current possesses energy, such a current can appear only through the expenditure of mechanical work or of some other form of energy.

FIG. 300.

Currents induced only when conductor cuts lines of force

348. Condition necessary for an induced E.M.F. Let the coil be held in the position shown in Fig. 300, and moved back and forth parallel to the magnetic field, that is, parallel to the line NS. No current will be induced.

By experiments of this sort it is found that an E.M.F. is induced in a coil only when the motion takes place in such a way as to change the total number of magnetic lines of force which are inclosed by the coil. Or, to state this rule in more general form, an E.M.F. is induced in any element of a conductor when, and only when, that element is moving in such a way as to cut magnetic lines of force.*

FIG. 301. E. M. F.

induced when a straight conductor cuts magnetic-lines force must cut

It will be noticed that the first statement of the rule is included in the second, for whenever the number of lines of force which pass through a coil changes, some lines of across the coil from the inside to the outside, or vice versa.

349. The principle of the electric motor. Let a vertical wire ab be rigidly attached to a horizontal wire gh, and let the latter be supported by a ring or other metallic support, in the manner shown in Fig. 302, so that ab is free to oscillate about gh as an axis. Let the lower end of ab dip into a trough of mercury. When a magnet is held in the position shown and a current from a dry cell is sent down through the wire, the wire will instantly move in the direction indicated by the arrow f, namely, at right angles to the direction of the lines of magnetic force. Let the direction of the current in the wire be reversed. The direction of the force acting on the wire will be found to be reversed also.

We learn, therefore, that a wire carrying a current in a magnetic field tends to move in

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FIG. 302. The principle of the electric

motor

* If a strong electromagnet is available, these experiments are more instructive if performed, not with a coil, as in Fig. 300, but with a straight rod (Fig. 301) to the ends of which are attached wires leading to a galvanometer. Whenever the rod moves parallel to the lines of magnetic force there will be no deflection, but whenever it moves across the lines the galvanometer needle will move at once.