LESSON EIGHT. THE PRINCIPLE OF THE TRANSFORMER.
The transformer is a device for raising or lowering A the electromotive force of an alternating current. In principle it consists of two insulated coils of wire called the primary and the secondary wound around an iron ring as shown in the illustration.
FIG. 22. Diagram showing the principle of a Transformer.
If the primary coil is connected to a source of alternating current it will rapidly magnetize and demagnetize the iron ring. The magnetic lines thus created will pass through the secondary coils setting up induced currents.
The ratio of the electro-motive force of the induced current to that of the primary current is in direct proportion to the ratio of the number of turns in the two coils. For example, if the secondary contains twice as many turns as the primary, its electro-motive force will be twice as great.
FIG. 23. Open and Closed Core Transformers.
Transformers are of two general types, the "open" core and the "closed" core. Closed core transformers are the most efficient. The open core transformer is similar in construction to an induction coil, the core being a straight bar, while that of the closed core machine is usually in the form of a hollow square or rectangle.
In practice, the cores of transformers are built up of laminations, usually of thin, soft sheet iron strips piled together and shaped so as to constitute a closed magnetic circuit of rectangular shape in order to avoid constructional difficulties incurred in making a ring.