Alarm clock (telephone bell)

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Fitted base plate of the desk telephone W48 from 1957 with a double-shell alarm clock

Alarm clock is the technical term for a telephone bell , the acoustic signal generator for incoming calls. In earlier telephones , such as the telephones of the Deutsche Bundespost , the alarm clock was an electromechanical bell. However, it can also be installed as an additional signal source outdoors, for example in the courtyard of a property.

From the 1990s, the electromechanical bells were largely replaced by electronic ones, often with adjustable ring tones .

task

Additional alarm clock for interiors, model WK 952

Electric alarm clocks have the task of converting electrical signals into acoustic signals. A distinction is made between direct current and alternating current alarm clocks. The mode of action is the same for both types. An armature movably arranged in front of the poles of an electromagnet is attracted when the current passes through the magnet coils . A clapper attached to the anchor strikes a bell shell. In general, AC alarm clocks are used as call organs. After an endurance test of 100 hours at the Central Telecommunications Office , the alarm clocks still had to be functional.

functionality

DC alarm clock

With the direct current alarm clock, a device is necessary that ensures that the anchor is not only tightened once for the duration of the ringing tone, but alternately tightened and released, so that the clapper is set in constant motion. This can be achieved in two ways, namely:

  1. by the attracted armature interrupting the circuit of its own magnetic coils, thereby dropping out and closing the circuit again ( Wagner's hammer ) or
  2. by short-circuiting the solenoid coils via a contact that is also connected to the armature (rarely).

In both cases, the attracted armature interrupts the flow of current in the magnet coils, and the armature drops out. This closes the current path again and attracts the armature again. This interplay is repeated as long as the alarm circuit is closed by the control process (bell button). DC alarm clocks are rarely used in telecommunications .

AC alarm clock

AC alarm clock of a
FeTAp 791 (1970s)

To control AC alarm clocks, the DC supply voltage of the telephone line is superimposed by an AC voltage (typically 25 Hz), the ringing voltage . In series with the alarm clock's electromagnet is a capacitor (typically 1 µF) which only lets through the AC voltage. In front of the pole pieces of the electromagnet there is an armature which is rotatably mounted in the middle and carries a clapper bar. By inserting small permanent magnets with a high field strength into the magnetic circuit , which is formed by the electromagnet core and the armature, the armature is resting in the attraction area of ​​the north or south pole of the electromagnet core. The electromagnetic coils are wound in such a way that, for example, with a positive half-wave of the alternating current at the free ends of the cores, the poles created by the permanent magnets are weakened in one leg and strengthened in the other. When changing the direction of the current, the reverse process occurs. As a result, the armature is attracted alternately from one pole or the other.

literature

Individual evidence

  1. ^ Educational sheets : »Telephone sets of the 71, 73, 75, 79 series« ; Issue 36/1983 No. 1, p. 4

Web links

Commons : Telephone bells  - collection of pictures, videos and audio files