FIG. 20. Motor Generator.
Each variety of dynamo winding has a certain advantage depending upon the condition of use.
In the case of alternating current dynamos, the field magnets are sometimes supplied from a separate dynamo called an "exciter." In other cases the dynamo is provided with two sets of windings, one connected to a commutator producing a direct current which excites the field coils and the other connected to a set of rings and supplying the alternating current.
In case a supply of either direct or alternating current is available and it is desirable to change the supply from direct to alternating or vice versa, it may be accomplished by employing a Motor-Generator. A motor-generator consists of an electric motor operating from the source of current supply on hand and driving a dynamo which supplies current of the kind desired.
A motor is exactly the reverse of a dynamo. If a current of electricity is passed into a dynamo, the armature will be dragged around by the mutual action of the currents flowing in the copper conductors and the magnetic field in which they lie. Such a device constitutes a motor and may be employed to do useful work.
Motors are classified as alternating and direct current machines accordingly as they are built to operate on either kind of current.
LESSON SEVEN. THE INDUCTION COIL.
The Induction Coil is an apparatus for producing currents of a very high electromotive force. It consists of a helix of large, insulated wire surrounding an iron core, and this again surrounded by a second coil consisting of many thousand turns of very fine wire carefully insulated. The inner or primary coil is connected in series with a battery, the circuit also including a device called an interrupter. The object of the interrupter is to make and break the primary circuit in rapid succession. Every time the current is turned on in the primary circuit, the primary coil creates a magnetic field which induces a current in the secondary in accordance with the laws of induction.
