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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.
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- 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