Construction of a Small Bell-Ringing Transformer
By A. E. Andrews
Part I—Fundamental Principles
The transformer in its simplest form consists of two separate and electrically independent coils of wire, usually wound upon an iron core.
Fig. 1—Two Coils on an Iron Ring
Figure 1 shows two coils, P and S, placed upon an iron ring, R. One of these coils is connected to some source of energy, such as an alternating-current generator, or an alternating-current lighting circuit, receiving its energy therefrom. The other coil is connected to a load to which it delivers alternating current. The coil of the transformer that is connected to the source of energy is called the primary coil, and the one that is connected to the load, the secondary coil.
The electrical pressure (voltage) at which current is supplied by the secondary bears a definite relation to the electrical pressure at which current is supplied to the primary. This relation, as will be explained later, is practically the same as the relation between the number of turns in the secondary and primary coils. If there are a smaller number of turns in the secondary coil than there are in the primary, the secondary voltage is less than the primary, and the transformer is called a step-down transformer. If, on the other hand, there are a larger number of secondary turns than of primary, the secondary voltage is greater than the primary voltage, and the transformer is called a step-up transformer.
The transfer of electrical energy from the primary coil to the secondary coil of a transformer is based upon the fundamental principles of electromagnetism and electromagnetic induction, and it will be necessary to investigate these principles before we can understand the operation of the transformer.
A magnet is a body, which, when freely suspended, assumes approximately a north and south position. The end of the magnet that points north is called the north pole, while the end that points south is called the south pole. The region surrounding a magnet is called a magnetic field. In this field the magnetism is supposed to flow along a large number of imaginary lines, called lines of force, and these lines are all supposed to emanate from the north pole of the magnet, pass through the medium surrounding the magnet and enter the south pole. The magnetic field surrounding a bar magnet is shown in Fig. 2. The strength of any magnetic field depends upon the number of these lines of force per unit area (square centimeter), the area being taken perpendicular to the direction of the lines.