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nected respectively to the binding posts 1 and 3, and to the binding post 2 is connected the ground wire from the lightning arrester a. The receiver r is connected between the binding posts 4 and 5,
usually by 2į feet of No. 18 B. & S. gage stranded twin conductors, separately insulated with a rubber covering and together overlaid with strong worsted or silk braid. These conductors generally terminate in solid tips, and the braid is usually extended
to connect either with the case or the magnetic system of the receiver so that there is no strain on the conductors. Two cells of battery o, the transmitter t, and the primary winding of the induction coil i are wired to the binding posts 6 and 7 while the switch arm s and the secondary winding of the induction coil i are connected between the binding posts 8 and 9. The bell c is mounted on the door m of the magneto box, which is open in Fig. 32 to show the interior connections. It is wired to two metal strips h and b, which, in turn, are soldered to the hinges of the door. On the inner side of each hinge is fastened a little spring which presses upon the opposite face of the hinge and insures good electrical contact with the bell circuit when the door is closed. The dotted lines in Fig. 32 indicate the circuits through the magneto generator and its automatic shunt, as shown in Fig. 29. These circuits, as already explained, are formed by the parts of the apparatus itself, and, therefore, are not a part of the present wiring; they are, however, indicated in the diagram to make clearer the connections to them from the lower hinge and the binding post 7.
The Lightning Arrester is used to protect the telephone set from lightning discharges upon the line wires, by providing for such discharges a more direct path to earth than that afforded by the telephone instruments. The Stromberg-Carlson carbon-block arrester shown in Fig. 33 is similar to the one indicated at a, Fig. 32. It consists of
two sets of carbon blocks, cd and es, held between the brass springs or clips m n and u v. The two carbon blocks in each set are separated by thin strips of mica perforated with holes, and the binding post r, with which the central clips and carbon blocks are in electrical contact, must be carefully connected to earth by as short and straight a conductor as possible. The binding posts b and h must be joined to the interior circuits of the telephone set and to the line wires as already explained. If, then, a lightning discharge comes in
on either of the line wires, its high voltage enables it to jump the short space between the carbon blocks and follow the ground wire to earth; this it does in preference to traversing the inductive circuit through the instruments. Telephone currents being of comparative low voltage cannot cross the space between the carbon blocks and, therefore, are not diverted from their course by the arrester.
The Magneto Bell in a series telephone set offers the only appreciable resistance through a station when the talking circuit is not in use. If the re
sistance of the bell windings is small, a number of series telephone sets as in Fig. 34 can be joined in series upon a line, and the ringing current from any one of them will operate all of the bells simultaneously. The magnet coils of ringers in series telephone sets are, therefore, wound to as low a resistance as possible; 80 ohms is considered standard practice, that is, 40 ohms per coil with the' 2 coils joined in series. Their cores are also made as short as possible to reduce any unnecessary impedance to the alternating current from the magneto generator. Although 4 stations are represented in Fig. 34, 2, 3, or more than 4 can be joined in series connection by following the method there shown; the grounded line wires at the 2 end stations will serve for
Fig. 34.—Method of Wiring Series Telephone Sets on a Grounded Circuit
the lightning-arrester grounds if the center and end binding posts on the telephone sets at these stations be joined together as indicated. Fig. 35 shows 4 series telephone sets wired in series on a complete metallic circuit. The ground is then used only for the lightning-arrester connections.
The Interior Wiring Between the Telephone Set and Line Wires is preferably done with No. 18 B. & S. gage rubbercovered stranded copper wires twisted in pairs, unless the conductors are to be much exposed, in which case it is advisable to use No. 16 B. & S. gage rubber-covered stranded wires. The rubber insulation on the No. 18 should be at least inch thick, and on the
Fig. 35.—Method of Wiring Series Telephone Sets on a Complete Metallic Circuit