Fig. 233.
The reason why there is a tendency for an electric current to flow in the wire CD when it is moved in the vicinity of a magnet is not fully known. There are several theories, all more or less complicated, and depending upon pure assumptions as to the nature of an electric current. For practical purposes it matters little what the reason is, the fact that current flows when there ts an electric pressure in a closed circuit, is the important thing, and it serves all useful purposes to know that current does flow, and that its direction and amount are always the same under similar circumstances. There are many facts in mechanics that are accepted and used practically, about which little is known as to their fundamental and primary causes, and this fact about motors and dynamos is, therefore, only one of many which all must accept without a full and complete explanation.
The intensity of the electric pressure, or electro-motive force, depends upon the velocity of revolution of the wire sections in the armature and upon the strength of the magnets, and the quantity of current depends upon the electro-motive force and upon the amount of the resistance in the circuit. Other things being equal, the current, flowing through a long small wire, or greater resistance, will be less than through a short, thick wire, or a less resistance.
Having seen that when a wire is moved in the vicinity of a magnet an electric pressure is produced which will cause a current to flow in a closed circuit, one can easily conceive of many ways in which, by combining magnets and wires so that there will be a relative motion between them, a current of electricity may be generated. In order to cause a continuous flow the relative motion must be continuous; and if the current is to be uniform the motion must be uniform.
Fig. 234.
Two electro-magnets are shown in [Fig. 234], in which the North pole of one magnet is near the South pole of the other, and the magnetic field between the two lies in the approximately straight lines between the two magnets, as indicated by the dotted lines. If the wire CD be moved across this field and its ends be joined, as by the metallic circuit CEFD, a current will flow in this circuit. The wire CD may be made to revolve around the wire EF, passing in front of one pole and then in front of the other pole, as in [Fig. 235]. The current in the circuit will pass in one direction when the wire is passing one pole, and in the other direction when it is passing the other pole. The connection between this elementary arrangement and the dynamo is easily recognized. In the dynamo a magnetic field is produced by electric magnets, called “pole pieces,” and a considerable number of wires similar to the wire CD are placed upon an armature so that they revolve in front of these poles. Each individual wire produces current first in one direction and then in another direction, as explained above; but if there be many wires there will always be the same number in front of the North, or positive pole, and the same number in front of the South, or negative pole, so that the total or resultant action is practically uniform, and may be made to produce a continuous current. Such a machine is the common direct current dynamo, or motor.
Fig. 235.