FIG. 29.

MAGNETIC AND ELECTRIC CURRENT.

showing that the lines of force run completely round the wire, and do not stand out in tufts. In fact, every conducting wire is surrounded by a sort of magnetic whirl, like that shown in Fig. 29. A great part of the energy of the so-called electric current in the wire consists in these external magnetic whirls. To set them up requires an expenditure of energy; and to maintain them requires also a constant expenditure of energy. It is these magnetic whirls which act on magnets and cause them to set as galvanometer needles do at right angles to the conducting wire.

MAGNETIC WHIRL

Now, Faraday's principle is nothing more nor less than this: that by moving a wire near a magnet across a space in which there are magnet lines, the motion of the wire, as it cuts across those magnetic lines, sets up magnetic whirls round the moving wire, or, in other language, generates a socalled current of electricity in that wire.

It is however necessary that the moving conductor should, in its motion, so cut the magnetic lines as to alter the number of lines of force that pass through the circuit of which the moving conductor forms part. If a conducting circuit-a wire ring or single coil, for example-be moved along in a uniform magnetic field, so that only the same lines of force pass through it, no current will be generated, see Fig. 30. Or if, again, as in

SURROUNDING

WIRE CARRYING

CURRENT.

MAGNETIC AND ELECTRIC CURRENT.

Fig. 31, the coil be moved by a motion of translation to another part of the uniform field, as many lines of force will be left behind as are gained in advancing from its first to its second position, and there will be no current generated in the coil. If the coil be merely rotated on itself round a central axis, like the rim of a fly-wheel, it will not cut any more lines of force than before, and this motion will generate no current. But if, as in Fig. 32, the coil be tilted in its motion across the uniform field, or rotated round any axis in its own plane, then the number of lines of force that traverse it will be altered, and currents will be generated. These currents will flow round the ring coil in the righthanded direction (as viewed by a person looking along the magnetic field in the direction in which the magnetic lines run) if the effect of the movement is to diminish the number of lines of force that cross the coil; they will flow round in the opposite sense, if the effect of the movement is to increase the number of intercepted lines of force.

If the field of force be not a uniform one, then the effect of taking the coil by a simple motion of translation from a place where the lines of force are dense to a place where they are less dense, as from position I to position 2 in Fig. 32, will be to generate currents. Or, if the motion be to a place where the lines of force run in the reverse direction, the effect will be the same, but even more powerful.

MAGNETIC AND ELECTRIC CURRENT.

FIG. 30.

CIRCUIT MOVED WITHOUT CUTTING ANY LINES OF FORCE.

FIG. 31.

CIRCUIT MOVED SO AS TO ALTER NUMBER OF LINES OF

FORCE THROUGH IT.

MAGNETIC AND ELECTRIC CURRENT.

We may now summarize the points under consideration and some of their immediate consequences, in the following

manner:

(1.) A part, at least, of the energy of an electric current exists in the form of magnetic whirls in the space surrounding the conductor.

(2.) Currents can be generated in conductors by setting up magnetic whirls round them.

(3.) We can set up magnetic whirls in conductors by moving magnets near them, or moving them near magnets.

NOTE.-As a matter of fact, it would be impossible to have a magnetic field exactly like Fig. 32; for, in the intermediate part, between the upper and lower fields, the magnetic lines would be of curved complex form.

MAGNETIC AND ELECTRIC CURRENT.

(4.) To set up such magnetic whirls, and to maintain them by means of an electric current circulating in a coil, requires a continuous expenditure of energy, or, in other words, consumes power.

(5.) To induce current in a conductor, there must be relative motion between conductor and magnet, of such a kind as to alter the number of lines or force embraced in the circuit.

(6.) Increase in the number of lines of force embraced by the circuit produces a current in the opposite sense to de

crease.

(7.) Approach induces an electromotive-force in the opposite direction to that induced by recession.

(8.) The more powerful the magnet-pole or magnetic field the higher will be the electromotive-force generated.

(9.) The more rapid the motion, the higher will be the electromotive-force.

(10.) By joining in series a number of such moving conductors, the electromotive-forces in the separate parts are added together; hence very high electromotive-forces can be obtained by using numerous coils properly connected.

(11.) Since the quantity or strength of the currents depends on the resistance of the conductors in the circuit, as well as on the electromotive-force, all unnecessary resistance should be avoided.

(12.) Approach being a finite process, the method of approach and recession (of a coil towards and from a mag