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This is a very early barretter and does not have a type designation as the later devices have. The barretter lamp uses iron wire and probably an hydrogen filling. The property of the barretter is a positive temperature coefficient as with a thermistor. As the current increases, the resistance also increases opposing the rise in current. In this way filament or heater chains could have uniform current flow in spite of variations in mains voltages. Most barretters were fitted into standard valve envelopes.

The thin glass tube envelope is 21 mm in diameter, and excluding the base pins, is 70 mm long.

Reference: Observation.