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induction coil must be connected in a local closed circuit. (4) When the talking circuit is not in use, the battery circuit must be opened. It is possible to satisfy these conditions with the signaling apparatus either in series with the line or bridged across it.
The Series Connection is shown in Fig. 28, b representing the magneto bell, & the magneto generator, and m and n the line wires. When the line is not in use for conversation, the receiver r
Fig. 28.—The Series Connection.
is hung on the end of the metal hook switch a, and its weight keeps a down so that it touches the contact piece c. The generator g is provided with a shunt s, which forms a low-resistance path around g when the generator is not in use. The shunt, however, is opened automatically when the crank handle of the generator is turned in the act of ringing the bell at the other end of the line.
Signaling current coming in at m, passes through the magnet windings of the bell b, the shunt s, the hook switch a, the contact c, and out at n, ringing the bell b. Condition (1) is, therefore, satisfied. With the exception of the shunt s being opened as already explained, and the armature winding of the generator introduced, the circuit through the apparatus shown remains the same when the generator g, is operated to signal a distant party, so that condition (2) is also satisfied. The party signaled by the ringing of the bell b removes the receiver r from the hook of the switch a and places it to his ear, whereupon a is drawn upward against the contact pieces u and v, as in Fig. 28, by means of a spring. This action leaves the receiver r connected to the line wires by the circuit m val y r n, and places the transmitter h, the battery e, and the primary winding p of the induction coil in the local closed circuit eu al ph, satisfying condition (3). Condition (4) is satisfied when the receiver r is replaced on the hook switch a, because the contact at u in the battery circuit is then opened.
The Automatic Shunt previously referred to in connection with the magneto generator, is shown in one form in Fig. 29. A coil of the armature winding is represented at [, upon the armature core e. One wire of the signaling circuit is connected to the armature shaft, and the other wire to the pin o which is secured to an insulated bushing a inserted in one end of the armature shaft. The shunt is in the form of a spring g, screwed at one end to the iron core of the armature and carrying at the other end a weight w. Owing to
the tension of the spring g, its free end normally presses against the pin v, short-circuiting the armature winding. When the armature is put in motion, however, centrifugal force causes the weight w to fly out, separating the spring from v and forcing it in contact with the stop p. As p is in connection with the iron armature core, the armature winding is thereby introduced in circuit and the shunt is opened. As soon as the armature ceases
Fig. 29.-Automatic Shunt of the Magneto Generator
to revolve, the spring again comes in contact with v and closes the shunt circuit. This device is often called a “centrifugal” shunt owing to its action being caused by centrifugal force.
The Hook Switch for a series connection is shown in one form in Fig. 30. It is pivoted at o and supported at the other end by the spring s, being thus free to move up and down through the slot in the box in which it is mounted. Its
action in making connection with the upper contacts w and v, and the lower contact c, when the receiver is respectively off or on the hook h, is already familiar to the reader. The hook switch is usually of brass, nickel-plated, and the springs are of steel with platinum tips. Platinum tips are employed because they do not corrode, but present a good wearing and contact surface.
Details of Wiring a Series Station.-In order that the telephone instruments indicated in the plan of
Fig. 30.-One Form of the Hook Switch
wiring, Fig. 28, be properly protected from injury and occupy as little unnecessary space as possible, it is customary to assemble them in a wooden case such as that shown in Fig. 31, and fasten the case to the wall. The apparatus thus grouped together is known as a telephone set or station; in the style of case shown it is designated as a solidback wall set. The wooden box a contains the magneto generator, magneto bell, and switch arm. The switch arm projects outside the box and holds the receiver. The semi-cylindrical iron case b
contains the induction coil and provides a support for the transmitter arm. The wooden box c holds the battery, which consists of two cells, and its sloping top affords a place for recording notes
received through the telephone. The set should be located in a convenient and accessible, yet out-ofthe-way place on a solid wall free from vibration; it should be fastened to the wall with screws at each of the four corners and at such a height that the mouthpiece of the transmitter when in its normal position will be about 5 feet above the floor.
The Wiring Inside of the Telephone Set
is done with No. 18 Fig. 31.-A Solid-back Wall B. & S. gage cottonSet
covered stranded copper wire, according to the diagram given in Fig. 32. In some sets the connecting wires are run in grooves in the backboards or bases and covered with hot bees-wax to exclude moisture. The conductors leading to the line wires are con