In Fig. 149 is shown the complete circuit of a bridging telephone.  The circuit given in this figure is for a local-battery wall set similar in type to that shown in Figs. 142 and 143.  A simplified diagrammatic arrangement is shown in the lower left-hand corner of this figure, and from a consideration of this it will be seen that the bell circuit across the line is normally completed through the two right-hand normally closed contacts of the switch on the generator.  When, however, the generator is operated these two contacts are made to disengage each other while the long spring of the generator switch engages the left-hand spring and thus brings the generator itself into the circuit.

[Illustration:  Fig. 149.  Circuit of Bridging Magneto Set]

Of the three binding posts, 1, 2, and 3, at the top of Fig. 149, 1 and 2 are for connecting with the line wires, while 8 is for a ground connection, acting in conjunction with the lightning arrester mounted at the top of the telephone and indicated at 4 in Fig. 149.  This has no function in talking or ringing, and will be referred to more fully in Chapter XIX.  Suffice it to say at this point that these arresters usually consist of two conducting bodies, one connected permanently to each of the line binding posts, and a third conducting body connected to the ground binding post.  These three conducting bodies are in close proximity but carefully insulated from each other; the idea being that when the line wires are struck by lightning or subjected otherwise to a dangerous potential, the charge on the line will jump across the space between the conducting bodies and pass harmlessly to ground.

NOTE.  The student should practice making simplified diagrams from actual wiring diagrams.  The difference between the two is that one is laid out for ease in understanding it, while the other is laid out to show the actual course of the wires as installed.

If the large detailed circuit of Fig. 149 be compared with the small theoretical circuit in the same figure, the various conducting paths will be found to be the same.  Such a simplified circuit does more to enable one to grasp the fundamental scheme of a complex circuit than much description, since it shows at a glance the general arrangement.  The more detailed circuits are, however, necessary to show the actual paths followed by the wiring.

The circuits of desk stands do not differ from those of wall sets in any material degree, except as may be necessitated by the fact that the various parts of the telephone set are not all mounted in the same cabinet or on the same standard.  To provide for the necessary relative movement between the desk stand and the other portions of the set, flexible conductors are run from the desk stand itself to the stationary portions of the equipment, such as the battery and the parts contained in the generator and bell box.