OSRAM BARRETTER ON GPO BASE, 1930's

Barretters are temperature sensitive resistors used for stabilising voltages in wireless receivers, usually powering the heater circuits, of AC/DC receivers.
Acting like a resistor but with constant current characteristics, a current over a particular range can be held constant over a range of varying voltage. A Barretter usually consists of an Iron wire held in Hydrogen filled glass envelope. Also used for many other applications.

Your comments:

• Barretters were also used in the MCVFT (multi carrier voice frequency telegraphy) units to limit the current through the output telegraph relay to prevent contact damage in the event of a short circuit on the output.
  I used to work on these units when I was a youth in training with the External Telecommunications executive (ETE), a division of the GPO, back in the mid 1960s. I was based at Electra House, Victoria Embankment, (the building was demolished around the late 1990s). I came across them (MCVFTs) again when working for the Civil Aviation Authority in the early 1970s.
  .......... Rodger Bean, Canberra, ACT, Australia, 27th of April 2021
  
  
• A similar device was used in telephone exchange switching devices, (final selector), to provide constant audio volume over different telephone line lengths.
  .......... Bill Buzby, London, 16th of February 2012
  
  

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