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operated exchanges, the latest perfected forms operate very well and their functions are performed with precision and accuracy. As is generally understood, in the present exchanges there is a pair of copper wires running to each subscriber, either directly or indirectly, and connec-. tion between any two parties is effected by joining the individual circuits while the wires are promptly separated at the end of the conversation. This is accomplished by the central station operators, and the subscriber has nothing to do with either establishing or breaking the connection.
The automatic systems must have some arrangement at each end of the line so the subscriber can call any party desired without any other aid and suitable mechanism must be provided so connection will be established with the desired party at a central station. The arrangement at the subscriber's end was formerly a series of push buttons, but at the present time a much more satisfactory and positive device does the calling. This is a numbered dial, such as shown at Fig. 127, attached to the base of a desk telephone of the usual form. The numbers around the edge of the dial range from 0 to 9, thus enabling any combination of figures to be selected. The neat dial is all that is visible at the outside of the instrument, but some very ingenious mechanism is hidden inside the telephone base and is actuated by the dial. At the edge of the
dial there is a little fixed hook or finger stop. In calling a number, the finger is placed in one of the openings in the upper dial, directly above a specific number, then the dial is rotated in a clockwise direction toward the fixed stop, until
its motion is arrested, when the finger is removed. Then the dial, which has been turned against the resistance of a torsion spring is returned to its starting place, but in so doing a toothed segment, rotating with the dial, makes a certain number of electrical contacts corresponding to the number at which the dial was released. Thus, if the dial was released when the finger in hole number 7
Fixed Finger Stop
of Rotation Rotary Dial
Fig. 128.—Method of Using Number Dial of Automatic
came against the finger stop, the contacts would send seven distinct impulses over the line to the central exchange, where a special electromagnetic selective device moves contacting fingers over suitable connections arranged so the desired circuit is brought in action. The calling of any number composed of more than one figure is accompanied by inserting the finger in the dial,
Fig. 129.–Automatic Wall Set. rotating it to the fixed stop and releasing it for each number in succession.
The operation of an automatic selector at the exchange will be understood by referring to Fig. 130, which is a simple selector or circuit finder for an exchange of ten subscribers. The subscriber's lines or circuits terminate in ten pairs of plates or “bank contacts” arranged on the arc of a circle and numbered from 1 to 10 respectively. A ratchet wheel, R, is arranged with its center at the center of the arc formed by the contacts and carries a pair of springs, W, technically termed
a wiper. The line for subscriber No. 1, for instance, by whom it is supposed this instrument will be used, is represented as being connected not only to the first pair of "bank contacts,” but also to the wiper, W, to the rotary magnets and to the release and rotary push buttons.
FIG. 130.—Operation of Automatic Telephone Selector.
Suppose that subscriber No. 1 wishes to connect to line No. 2. To do this, he presses twice (theoretically speaking) the rotary button (Rot. B) in the drawing located at his telephone. This closes the circuit from the battery through the rotary magnet. Every time the circuit is com