tracing circuits be acquired. In later chapters of this manual which deal with actual telephone circuits, the theory and
function of each part will be explained as soon as the part is introduced, and the relation of each part to the others in the
circuit will be presented by means of text and diagrams.
b. Types of circuit diagrams (fig. 21). Three basic types of circuit diagrams--pictorial, wiring, and schematic--
will be used extensively in this manual, and now will be described. In addition, frequent use will be made of block
(1) An example of a pictorial diagram is shown in A. This is a picture drawing of the actual physical layout or
assembly of the component parts of a circuit, showing the parts either as they appear to the eye, or in a form which
emphasizes some feature of their operation. The parts may be photographs of equipment, if they are arranged to show the
relationships among them. Pictorial diagrams are useful particulary to people untrained in the theory of operation of the
circuits they illustrate.
(2) B is an example of a wiring diagram. This type is used primarily to show a wireman or serviceman the proper
hookup for a piece of equipment. The emphasis in wiring diagrams is on the connection of cables and other wires to
appropriate terminals, not on the operation of the circuit.
(3) The schematic diagram, shown in C, is not a lifelike drawing of the component parts of a circuit, or a means
for indicating their connection. Instead, standard, conventional symbols are used, and the position of the symbols in the
diagram does not necessarily correspond to the location of the parts in the actual equipment. Schematic diagrams usually
are more compact and easier to trace than pictorial diagrams, and they are used more often. They make it possible to
present a more logical explanation of the voltage and current relationships in electrical circuits than is possible with other
types, and they allow the emphasizing of important features of the circuits. Schematic diagrams will be used extensively
in explaining the operation of the telephone circuits discussed in this manual.