is kept still and the coil thrust over it similar currents will be induced in the coil.  All that is necessary is for the wires to cut the lines of magnetic force around the magnet, or, in other words, the lines of force in a magnetic field We have seen already that a wire conveying a current can move a magnetic pole, and we are therefore prepared to find that a magnetic pole moved near a wire can excite a current in it.

Figure 38 illustrates the conditions of this remarkable effect, where N and S are two magnetic poles with lines of force between them, and W is a wire crossing these lines at right angles, which is the best position.  If, now, this wire be moved so as to sink bodily through the paper away from the reader, an electric current flowing in the direction of the arrow will be induced in it.  If, on the contrary, the wire be moved across the lines of force towards the reader, the induced current will flow oppositely to the arrow.  Moreover, if the poles of the magnet N and S exchange places, the directions of the induced currents will also be reversed.  This is the fundamental principle of the well known dynamo-electric machine, popularly called a dynamo.

Again, if we send a current from some external source through the wire in the direction of the arrow, the wire will move of itself across the lines of force away from the reader, that is to say, in the direction it would need to be moved in order to excite such a current; and if, on the other hand, the current be sent through it in the reverse direction to the arrow, it will move towards the reader.  This is the principle of the equally well-known electric motor.  Figure 39 shows a simple method of remembering these directions.

Let the right hand rest on the north pole of a magnet and the forefinger be extended in the direction of the lines of force, then the outstretched thumb will indicate the direction in which the wire or conductor moves and the bent middle finger the direction of the current.  These three digits, as will be noticed, are all at right angles to each other, and this relation is the best for inducing the strongest current in a dynamo or the most energetic movement of the conductor in an electric motor.

Of course in a dynamo-electric generator the stronger the magnetic field, the less the resistance of the conductor, and the faster it is moved across the lines of force, that is to say, the more lines it cuts in a second the stronger is the current produced.  Similarly in an electric motor, the stronger the current and magnetic field the faster will the conductor move.

The most convenient motion to give the conductor in practice is one of rotation, and hence the dynamo usually consists of a coil or series of coils of insulated wire termed the “armature,” which is mounted on a spindle and rapidly rotated in a strong magnetic field between the poles of powerful magnets.  Currents are generated in the coils, now in one direction then in another, as they revolve or cross different parts of the field; and, by means of a device termed a commutator, these currents can be collected or sifted at will, and led away by wires to an electric lamp, an accumulator, or an electric motor, as desired.  The character of the electricity is precisely the same as that generated in the voltaic battery.