Inductors
Panel de conmutación manual con operadoras, conjuntores y cordones que permitían conectar llamadas antes de la automatización.
The inductor L is designed to prevent signal currents from completing their circuit through the battery, which would keep them from reaching their destination. In this system, direct current is present on the line. However, when the number of active stations is very large, the advantage of powering all of them with a single battery more than compensates for the inconvenience of having direct current in the line. Practically all current telephone installations follow this basic model.
In the diagram, H represents the telephone hook, the device used to hang or rest the handset. The pressure exerted by the handset pushes the hook down, interrupting contact C. This prevents the device from drawing current while not in use.
Principles of Telephone Switching
For all telephone subscribers in a city to communicate with one another, they must be connected to a central office, where the interconnection between the calling subscriber and the called subscriber is established. There are two general types of central office: manual and automatic.
In a manual central office, all subscriber lines terminate at a large switchboard within reach of a group of operators. Each termination is equipped with a special connecting jack and a signaling system that activates a lamp when a subscriber picks up the handset. The operator, equipped with a microphone and receiver, has several connection cords, each ending in two plugs that are inserted into the jacks of the lines to be interconnected.
When the operator notices a call, she inserts the answering plug of one cord into the corresponding jack. She then contacts the calling subscriber through a dedicated switch and requests the number of the desired subscriber. Next, she inserts the calling plug of the same cord into the called subscriber’s jack, sending a ringing current to that device through a second switch. Once the called subscriber answers, the ringing current is interrupted, and the connection is established.
Her workstation also includes a set of monitoring lamps allowing her to detect when the called subscriber has answered and when the conversation ends.
If the number of subscribers exceeds one hundred, the work is divided among several operators. Each line then terminates in multiple jacks. One of these is the local jack, the only one equipped with the ringing system. The remaining jacks, known as general jacks, are distributed within reach of all operators and allow any operator answering a call via a local jack to complete it using the general jack nearest to her. It has been determined that an operator can usually handle between 50 and 100 local jacks.
To illustrate the scale: imagine a city with 9,000 subscribers. If each operator manages 100 local jacks, the service requires 90 operators. Before each operator is a panel containing the 100 local jacks she manages and 3,000 general jacks. Due to the arrangement of workstations, each operator also has access to the 3,000 general jacks on the panel of the operator to her right and the 3,000 on the panel to her left. Thus, every subscriber has one general jack accessible to every operator. In total, the central office would contain 3,000 × 90 = 270,000 general jacks.
The maximum practical capacity of a manual switching office is estimated at 10,000 subscribers. Therefore, large cities require several interconnected central offices. When an operator receives a request to connect with a subscriber in another office, she transfers the call to the appropriate office, where another operator completes it.
A similar procedure is used for long-distance communications. In small towns with a single central office, part of the switchboard is dedicated to long-distance connections, and requests are forwarded by regular operators. In systems with several central offices, a special long-distance office handles these calls. Long-distance connections may require the simultaneous intervention of several such offices across multiple cities, following predetermined routes. Long-distance operators rely on routing charts showing the most common paths, allowing them to direct calls efficiently. When a route is unusual, the request is transferred to a specialized operator.
