TELEPHONE No. 21 | ||||||||
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The press button on top of the case was depressed when calling with a ringing generator. The button could also be used to regulate ringing signals whilst the generator handle was turned continuously. This Telephone provides all the connections made by the two-lever form of P.O. Telephone, and superseded the "Telephone, P.O. for Granular Transmitter" and the "Telephone, P.O. for Granular Transmitter, "A" (Telephone No. 17). This meant that a second lever could be installed and the telephone was then called (Telephone for Granular Transmitter (with two levers)..Previously known as the "Telephone for Granular Transmitter 'C' (GTC)" it can be found in the 1906 Rate Book. By 1911 the telephone was renamed as the Telephone No. 21. Whilst similar to the Telephone No. 17 and No. 19 it differs in respect to the switch hook mechanism, which is split into two sections. The switch consists of a moveable contact arm and 5 springs, which equate to two changeover contact sets.. The Switch lever is divided into two parts, the inner part being insulated from the outer. When the Receiver is in its rest, spring B is in electrical contact with spring A through the lever, in the speaking position B and C are electrically connected (See diagram below).There is no bell in the telephone so an external bell must be provided. A Deckert transmitter was used originally but were later converted to a transmitter inset; the "Transmitter No. 3", which was fitted in a Case, Inset, Ebonite. The telephone can also have an additional receiver fitted ("Receiver, Bell D" with a "Cord, Flexible No. 222"). This will hang on the right hand side of the telephone on a fixed hook called an "Arm, Bell Receiver". The GPO Stores Rate Book 1928 & 1933 advises that this telephone was also used on Wallboards, Telephone, with fire-signal, London Fire Brigade and M.F.B. Note the oval brass number plate. Each bell and telephone carried an inventory number for identification. Around 1907 the Post Office realised that numbering each telephone was too much effort for too little usefulness and stopped the practice. The phone was powered by a Battery, Leclanche, 4 Cell, but if fitted in the cabinet at a call office installation then a Battery, Leclanche, 2 Cell would be provided. This telephone superseded the Telephone No. 17. This telephone was not available in the 1946 Rate Book. This telephone was mainly used in a call office installations and would have been mounted on a wallboard (see Diagram TL276). The telephone was also available in 1910 on a Wallboard
to diagram CBS 92B. Telephone included (1906):- Telephone includes (1910, 1911, 1912, 1913 and 1915):- Telephone includes (1933):- Depending on the use of the telephone, a Bell, Magneto 1000ohms or Bell, Trembler with a Relay, F (100ohms or 1000ohms) would be associated with the telephone. Circuit Diagram - N121.
Telephone No. 21
The above setup is used on a coin box installation where there is one Telephone No. 21 fitted at the counter and one fitted in a silence cabinet. The counter telephone is installed as shown above and includes a "Switch, 6 Terminal, 2 position", a "Bell, Magneto, 100ohm" and a "Bell, Trembler, Circular, 3 Terminal, 25ohm". The Magneto bell signals incoming calls and the Trembler bell signals when line is cleared. The cabinet also has a coin box connected the telephone. This telephone was also used when a Call Office Switchboard was installed. It supplied between 2 to 5 circuits. These telephones could also be used in many call office
installations including where both Post Office and NT trunks are presented and
shared by the subscribers apparatus with the Trembler bell signalling when the
customer rises from the cabinet seat and vacates the booth. Additional Pictures
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Last revised: November 03, 2025FM | ||||||||
This is a wooden wall telephone used in 
