Fig. 149. Circuit of Bridging Magneto Set
[View full size illustration.]

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.

Fig. 150. Circuit of Bridging Magneto Desk Set
[View full size illustration.]

In Fig. 150 is shown the circuit of the Stromberg-Carlson magneto desk-telephone set, illustrated in Fig. 145. This diagram needs no explanation in view of what has already been said. The conductors, leading from the desk-stand group of apparatus to the bell-box group of apparatus, are grouped together in a flexible cord, as shown in Fig. 145, and are connected respectively to the various binding posts or contact points within the desk stand at one end and at the base of the bell box at the other end. These flexible conductors are insulated individually and covered by a common braided covering. They usually are individualized by having a colored thread woven into their insulating braid, so that it is an easy matter to identify the two ends of the same conductor at either end of the flexible cord or cable.