Wireless Telegraphy and Telephony Simply Explained / A Practical Treatise Embracing Complete and Detailed Explanations of the Theory and Practice of Modern Radio Apparatus and Its Present Day Applications, Together With a Chapter on the Possibilities of Its Future Development

by Alfred Powell Morgan

FIG. 39.—Open and closed core transformers.

The transformer is acknowledged to be the best practice as a means of stepping up the voltage of a circuit for wireless telegraph purposes.

Alternating current is necessary to operate a transformer. There are two distinct types of transformers known as the "open" and "closed core" accordingly as the shape of the latter is straight like that of an induction coil or in the form of a hollow rectangle. The closed core transformer consists of two coils of insulated wire, forming a primary and a secondary, wound upon a rectangular core like that shown in Fig. 39B. The core is built up of sheets of iron called laminations, to reduce the heating and increase the efficiency of the machine.

FIG. 40.—Lines representing direct and intermittent direct currents.

As noted above currents are only induced in a coil when the magnetic field is changing. The interrupter is employed to rapidly "make" and "break" the circuit. Every time that the circuit is made the primary coil creates a field and every time it is broken it is destroyed. A direct current is a current which passes in one direction only. It may be represented by a straight line as A in Fig. 40. Its voltage is usually very constant and does not vary greatly. In the case of electric lighting circuits the normal voltage is usually 110. If an interrupter is included in the circuit the current may be represented by a broken line, the spaces corresponding to the periods when the current is "broken" and the lines to the periods it is flowing. The interrupter creates an intermittent direct current.

FIG. 41.—Diagram representing alternating current.

An alternating current is one which reverses its direction and passes first one way and then the other. It may be represented by the curved line shown in Fig. 41. It starts at zero and rises to a maximum, gradually dying away to zero, then passes in the opposite direction, rising to a maximum and dying away again. This is repeated a definite number of times per second; when the current rises from zero, reverses and returns to zero it is said to have passed through a cycle. From a to c represents a cycle—from a to b is an alternation. The usual frequency of commercial alternating currents is 60 cycles or 7200 alternations per minute.