Telegraph relay

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Telegraph relay, built before 1970
Telegraph relay
Switching center subscriber dial-up telegraph network TW 39, rack frame with selection and transmissions

A telegraph relay or T-relay was a special component in electromechanical communications technology . It was characterized by a very high sensitivity and almost no bounce .


T-relays were the most important part of a telex exchange . In contrast to the robust, slow switching relays, which were responsible for establishing the connection, it was in the signal path and regenerated the telex characters received. The transmission speed used was 50 baud , so a single bit lasted 20 msec. These short impulses had to be transmitted mechanically without errors and with little distortion . Due to the high switching speed, T-relays were only allowed to have a small moving mass, there was only one changeover contact.

Because of its good switching times and very good response sensitivity, this relay was also used in alternating current transmissions for the telephone network. It had the task of picking up the dialing pulses on the receiving end.

Although these relays were precision devices, they had a switching capacity of 100 watts. The voltage to be transmitted was referred to in telegraphy as double current, it was +/- 60 volts.

Mode of action

T-relays were polarized relays, so they could take different positions of rest even without electricity. This was achieved by a second magnetic circuit with a permanent magnet . The actual contact was part of both magnetic circuits, both the electro- magnetic and the permanent-magnetic.


A T-relay was a highly precise, fine mechanical component. Each relay has been manually adjusted for the lowest possible distortion. To secure against adjustment by mechanical vibrations during operation, the screws were then glued with paint. A protective cap served as protection against dust.

Subdivision of the T-relays:

  • neutral (not polarized), rest position = central position.
  • polarized, rest position depending on location and number of permanent magnets:
    • unipolar, rest position = one of the two contacts
    • bipolar, rest position = both contacts


  • Herbert Petzoldt: The telecommunication relay and its circuit. CF Winter'schen Verlagshandlung, 1952.


  • Kurt Pribich, Harald Gessinger, Helmut Haslinger: Components of communications technology: a textbook and reference book for all communications technology . 10., corrected u. erg. ed., Kohl and Noltemeyer Verlag, Dossenheim (Heidelberg) 1980, ISBN 3-88173-001-X .

Web links / references

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