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of the applications of electric currents utilize the fact that a current produces a magnetic field. The electric bell, the telegraph sounder, the telephone, all dy

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namos, and all electric motors depend on the magnetic action of electric currents.

The best way of representinga magnetic field is by means of magnetic lines. In the case of a current in a wire these are endless lines surrounding the wire. Fig. 244 is a photograph of the lines as traced out by iron filings, the wire being perpendicular to the plane of the paper, passing through at the center of the figure.

FIG. 244

In section 347 the direction of a magnetic field was stated to be the direction in which the north-seeking pole of a compass needle points. Using a compass needle in the field produced by a current, we find that when the current flows in one

direction, the needle points one way around the wire; when the current is reversed, the direction of the needle is reversed. In Fig. 245 the direction of the needle is shown in several positions when the current is flowing up* the wire. In all cases the needle points in the same direction as the magnetic lines around the wire.

FIG. 245

A very simple rule for the direction of the magnetic field around the wire is the following: When a person, actually or in imagination, grasps a wire with his right hand, with the thumb lying along the wire and

* By general agreement the current from a dry cell is said to flow away from the positively charged carbon terminal, which is usually the one in the center, and back to the cell at the negatively charged zinc, or outer, terminal. In the case of a gravity cell it flows away from the copper and back to the cell at the zinc electrode.

pointing in the direction of the current, the fingers will point around the wire in the same direction as the magnetic field.

This rule is very useful in many instances. For example, one can tell by a compass the direction of the field around S

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FIG. 246

FIG. 247

the wire, for the compass will point in the direction of the field. Knowing, then, the direction of the field, one can at once, by the right hand, determine the direction of the current.

Figs. 246 and 247 show the character of the magnetic lines produced by a current in a circular coil and in a helix. The direction of

the current is marked in each case. By means of the rule just given, the student should verify the directions of the fields shown in the figures.

If a bar of iron were inserted through the coil of Fig. 246 or the helix of Fig. 247 when a current is flowing through the wires, the iron would become magnetized by induction (see

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FIG. 248. A door-
bell magnet

magnetic induction, sect. 354). The field of the iron magnet would be added to the field of the coil, with the result that a very strong magnetic field would be obtained. A magnet produced in this way is called an electromagnet. If the core is a short piece of soft iron, it will retain its magnetism only while the current is flowing.

Fig. 248 shows a sketch of the magnet of a common doorbell. There are two connected coils, or spools, of wire, B and C, with an iron core I. The current goes around these connected coils in such

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as the circuit is broken the magnet loses its magnetism, and the spring M pulls the armature back. Then the circuit is closed again and the operation repeated.

Fig. 249 shows in section a type of magnet used in a telephone receiver. Two coils, A and B, are mounted on a magnet, M, near its poles. When a varying current flows through the coils, the attraction on the steel diaphragm D will change as the intensity of the current changes. The diaphragm will thus vibrate and give out sound-waves which will have the same frequency as the variations in the electric current. Fig. 250 shows a vertical and a horizontal section of a lifting-magnet, such as is used in lifting masses of iron or steel in yards and factories. The frame is of iron with coils of wireCC wound around the central core.

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FIG. 250. A liftingmagnet

406. Mechanical force acting on a wire carrying current in a magnetic field. If a coil of wire is suspended near the pole of a magnet, and a current is sent through it, there will be forces acting on the coil which tend to rotate it; if it hangs as shown in Fig. 251, it will tend to turn until it faces the pole. There is therefore a mechanical force acting on a coil of wire through which a current flows when a magnet is brought near. This is also easily shown by the simple experiment of the floating cell. A small test tube containing acidulated water is pushed through a large flat cork. A coil about three inches in diameter and consisting of a hundred or more turns of fine copper wire is connected at one end to a strip of zinc, and at the other end to a strip of copper. The coil A and the strips C and Z are bound with tape or twine so that the coil will hold the shape shown in Fig. 252. When the strips of zinc and copper are dipped down into the acidulated water, a current will flow around the coil. If the cell and coil

N

FIG. 251

float on water in a dish and if a pole of a magnet is brought near the coil, the action of the mechanical forces is plainly seen.

The tendency of a coil to rotate when placed in a magnetic field has many applications. The ordinary galvanometer, which is

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used in the laboratory to measure or detect currents, has a coil of wire (Fig. 253) hung

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between the two poles of a magnet. This coil is suspended by a fine wire which serves to carry the current, the circuit being completed by another fine wire leading from the bottom of the coil. When a current flows around the coil, the latter tends to turn so that its plane will stand perpendicular to the magnetic field between the poles; if the current is reversed in direction, the coil will turn in the opposite direction. The mechanical construction of galvanometers has been so highly developed that they are now extremely sensitive; that is, a perceptible rotation will be produced by a very small current.

The ammeter (the name of which is an abbreviation of amperemeter) is, as the term implies, an instrument used for measuring the current in amperes. The ordinary types of direct-current instruments contain a movable coil hung between the poles of a magnet. In principle the ammeter is like the galvanometer just described. Many electric motors work in a similar way: the armature, which contains coils of wire, is placed in a strong magnetic field, and rotates when a current flows through it.

The student should not be content with the meager information contained in this chapter about the mechanical effects of currents. It should be obvious that this is a very important part of the field of applied electricity. The topic is discussed more fully in Chapter XXXII.

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