We have no details on this Barretter. Measurements show that the iron wire connects between pins 2 and 7 - the standard heater connections for octal valves. The cold resistance is 5.9Ω.
Ralph has found details of a BU29/4 in edition 7 of the Wireless World data book. Neither is listed in the eighth edition. The Type designation seems to suggest the rating. The best guess is that a current of 0.285 Amps is stabilised for voltage drops of between 2.5 - 6.0 Volts.
The wire is looped over a single central support.
The wire is a single strand.
Thanks to Ralph Rayner for donating this exhibit.
The wide glass tube envelope is 30 mm in diameter, and excluding the IO base pins is 83 mm tall.
