Additional Material

Read the text and get its central idea. Describe the moving-coil galvanometer and compare its construction and uses with those of the moving-coil ammeter.

Moving-Coil Galvanometer

When a current flows in a coil of wire, the coil will ex­hibit many of the properties of a magnetic needle. If such a coil is suspended between the poles of a permanent magnet, it tends to turn, so that the lines of force due to the coil and those due to the permanent magnet will lie in the same di­rection. This gives a means of measuring a current by observ-

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ing the deflection that it produces if it flows in a coil о wire between the poles of a magnet. The magnet NS (Fig. 11 is usually made in the shape of a horse­shoe so that it will be as strong as possible. The coil is wound on a very fine phosphor bronze wire between the poles of the magnet. The bottom of the coil is connected to a binding post by means of a fine wire or ribbon wound in the form of a spiral. The current to be measured enters the coil through the wire by which it is suspend­e d and leaves through the wire or ribbon spiral at the bottom of the coil. Of course, the direction of the current may be reversed. When there is no cur­ rent in the coil of the galvanometer, the plane of the coil lies in the plane of the poles N and S of the magnet. When a current is pro­duced in the coil, it acts like a small magnet, with its axis perpendicular to the plane of the figure. The coil tries to turn itself so that its north pole should point in the direc­tion of the south pole of the permanent magnet and its south pole in the direction of the north pole of the permanent mag­net. The coil turns until the restoring torque, due to the tor­sion in the suspension, is as great as the torque produced by the current in the coil. Then the coil comes to rest, and the amount that it has turned is a measure of the current in the coil. A mirror is fastened to the coil so that its deflec­tion can be read with accuracy. The deflection is nearly proportional to the current in the coil.

UNIT 19

Text. Electromotive force

When free electrons are dislodged from atoms, electrical energy is released and made available to do work.¹ Chemcal reaction, friction, heat and electromagnetic induction will cause electrons to move from one atom to another. Scien­tists proved electrical energy to be released from matter by chemical reaction (batteries), heat (thermocouples), electro­magnetic induction (generators), and friction (static genera­tors). Whenever energy in any form is released, a force is developed. Electrical energy being released, a force called electromotive

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force (e.m.f.) is developed. An e.m.f. is pres­ent, then, whenever free electrons are moved from atoms, any of the above named methods being used to produce such electron motion.

If the force exerts its effort always in one direction, it is called direct; the force changing its direction of exertion periodically is referred to as alternating.

The chemical reaction in a dry cell produces a negative charge or potential on the zinc. This charge being always negative, the e.m.f. is unidirectional (one way). Heat and friction, too, are sources of unidi-rectional force. Electromagnetic induction, however, is certain to produce an alternating force.

I f the south end of a bar magnet (as the diagram shows, see Fig. 12) is passed into a coil of wire connected to a force-measuring instrument (voltmeter), the meter needle will move in one direction. If the south pole of the magnet is withdrawn from the coil, the needle will move toward the opposite side of the meter, thus show­ing the force to be alter­nating. The direction of force effort is seen to be dependent upon the di­rection in which the field is cut. The magnitude of the electrical force

depends on the conditions at the source, such as the num­ber of magnetic lines of force cut per unit of time.

In the battery, the determining factors are kinds of elec­trolytes and the kind of metal used for the plates. The com­mon dry cell is found to develop 1.5 volts of electrical force regardless of the size of the cell. Large amounts of force can be obtained only by putting many cells in series.

The force developed by the generator depends on the num­ber of coils in the armature, on the speed of the armature, and on the strength of the magnetic field from the field mag­nets, i.e., the number of lines of magnetic force cut by a coil per second. The volt is known to be the unit of measure for electrical force.

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