Click here for Transmitters
Transmitter Inset No. 2

This transmitter is a solid back type but is constructed in a cheaper fashion.  It is a capsule type.

It would be used on C.B. telephones.

Transmitter Inset No. 3

Early capsule transmitter used in Telephone No. 28 and other Local Battery telephones.

Used by the Transmitter No. 10.

Click here for more information

Transmitter Inset No. 9

Used on Electrophone Stage Circuits.  Installed in a Transmitter No. 16.


Transmitter Inset No. 10

Carbon granule transmitter introduced in the early 1930's and used in all Bakelite handsets, later Candlesticks.

The mouthpiece cover of the Mark 234 was fixed by 4 screws (these were made in the GPO experimental shop).
The Mark 235 had a graphite electrode and the cover was fixed in place by a concealed spring.

Introduced in 1929 and superseded by Transmitter No. 13 in 1939.

Mark 235 may have the TMC number 4006A on it.

Transmitter Inset No. 13

Carbon granule transmitter used in all Bakelite handsets and some early 700 types.

There appears to be two variants the /1 was all metal construction, whilst the /2 had the protruding back part made of plastic.

Transmitter Inset No. 13A was used in the Transmitter No. 23.
Transmitter Inset No. 13AA was used in the Transmitter No. 23A.

Transmitter Inset No. 13C was used initially on 700 type telephones and is identifiable by a screw-nut, located on the spring plate, on the rear of the unit.  There is a also a nut on a threaded pin pushed into the centre hole.

Superseded the Transmitter Inset No. 10.
Superseded by the Transmitter Inset No. 16 for 700 type telephones.

Introduced in 1939.

Ericsson Code No. N7752L (No. 13C).

Click here for an article from the Ericsson Bulletin

Transmitter Inset No. 15

Carbon granule transmitter used in Trimphone models and headsets.

The rubber grommet on the front is removed when used in Trimphones, but retained when used with Headsets No. 1 and 2.

Transmitter Inset No. 16

Carbon granule transmitter in general use from around 1960.  

Transmitter Inset No. 16 was originally fitted to all 700 type telephones.

Click here for a description of the transmitter

Superseded by Transmitter Inset No. 21.

Ericsson Code No. N7755A1.

There were numerous versions.  The Mark 3 can be identified by the number 3 after the year of manufacturer and red sealing compound will be evident.  Mark 3 insets were not to be used in public kiosks.

Transmitter Inset No. 21A

Superseded Transmitter Inset No. 16.  These transmitters were electronic and require line current to operate.

No. 21B was introduced at the end of 1982 and incorporated RFI suppression circuitry.

Click here for more information



BACK Home page BT/GPO Telephones Search the Site Glossary of Telecom Terminology Quick Find All Telephone Systems

Last revised: January 23, 2022