Reading of Schematic Diagrams.
The ability to trace and understand the schematic diagrams of telephone circuits can be obtained rapidly if the
problem is approached in an intelligent manner. Do not attempt to memorize complicated diagrams. The principles and
procedures followed by telephone men, described below, should be followed to acquire skill in reading and understanding
a. Learn the electrical principles underlying the operation of the particular circuit. This includes a knowledge of
the kinds of current flowing in the various parts of the circuit, the voltage across the various parts, and the power
dissipated in the circuit.
b. Memorize the symbols for the component parts of telephone circuits. These symbols will be introduced at the
time the operating principle of each part is explained. Learn to identify the symbol with the actual appearance of the part.
c. Break down a complex circuit into a number of simpler circuits. Frequently, certain small groups of parts
form relatively simple units within a complex circuit. For example, a diagram of a complete telephone system can be
broken down into a transmitter circuit, a receiver circuit, a ringing circuit, relay circuits, and several other smaller circuits.
Learn to recognize these small groups as units, and to relate these units to the others. In the following pages, complete
circuit diagrams will be built up step by step; and frequently, as each new smaller unit is introduced, its position in the
circuit will be emphasized by the use of heavier lines than those in the rest of the circuit. Take advantage of this, not only
to learn the function of the unit itself, but to understand its relation to the rest of the circuit.