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neto bell b is permanently bridged across the line wires and, being thus continuously in circuit, must offer a high impedance to the talking current so as not to divert it from its proper course. The ringer coils are therefore wound to a high resistance, generally 1,000 or 1,600 ohms, that is, 500 or 800 ohms per coil, on comparatively long iron cores. As far as the connections for the signaling apparatus are concerned, it is thus seen conditions (1) and (2), previously given, on page 37, are satisfied.


Fig. 45.—Hook Switch with Horizontal Spring Contacts

The hook switch z is connected permanently with both windings of the induction coil by the wire 0. When the talking circuit is in use, the receiver is off the hook switch, permitting the latter to be forced up by the spring 1 and make contact at u v; this action connects the receiver to the line wires by the circuit m 4 r 5 9 28 0 vn, and the transmitter, battery, and the primary winding of the induction coil are connected in the local closed circuit h 6 u o 7 pt, thereby satisfying condition (3). When the talking circuit is not in use, the receiver is on the hook switch; the weight of the receiver then overcomes the force of the spring 1 and, by pulling the switch z down, opens the contact made at u v and thus opens the battery circuit, satisfying condition (4).

Different forms of the hook switch for a bridging connection are shown in Figs. 45 and 46. In the Kellogg hook switch, Fig. 45, the contacts are formed by a series of German-silver springs at a, insulated from each other, and from the frame and the hook lever r, by hard rubber. The contact points are of platinum, riveted to the springs. The main spring b is also of German silver, and is fastened so as to act on the lever r through a short moving distance. In the Swedish-American desk-set hook switch A, Fig. 46, the contact springs are mounted vertically at c, and the main spring has the form shown at d. The principles of operation involved, however, are the same as previously described. Connection with the contact springs is made by the flexible wire cord shown entering the base s through the hard-rubber bushing e; and when the parts are assembled as shown at B, Fig. 46, the skill of the designer is appreciated.

Details of Wiring a Bridging Station.—The wooden box shown in Fig. 31 for housing the telephone apparatus of a series station would serve equally well for holding the apparatus of a bridging station, so that as far as outer appearances are concerned the two kinds of telephones may be similar. The wiring inside of the bridging tele

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Fig. 46.—Hook Switch with Vertical Spring Contacts as Used in a Desk Telephone Set


phone set, however, differs considerably from the former 'case, as already noted, and is shown in detail in Fig. 47. Particulars relating to the kind and size of wire to use, the connections for the

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Fig. 47.—The Inside Wiring of a Bridging Telephone Set

protective apparatus, and the wiring up to the line wires are identical with those already given for the series station, and the reader is referred to them for further details.

An Extension Bell in a Bridging Station is just

as convenient and as readily connected as in a series station. The two wires from the binding posts m and n of a bridging extension bell, Fig. 48, would be connected respectively to the binding posts 1 and 3, Fig. 47. The ringer coils in the Acme extension bell shown are wound to a resistance of 1,000 or 1,600 ohms to correspond with those in the main telephone set.

The Wiring for a Desk Set is shown in Fig. 49.

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Fig. 48.—An Extension Bell for a Bridging Station

The line wires m and n are brought to the connecting rack r, on which is usually mounted the induction coil ac. The primary winding of this coil is denoted by a, and the secondary winding by c. The battery b is generally placed near r, and the desk set, comprising the telephone stand s, and the box d containing a magneto bell and generator, is placed where desired, the telephone on the desk, and the box screwed either under it or at its side.

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