The GEC 304 Barretter is a current stabilising device for controlling the current in series heater chains of AC/DC mains equipment. It acts a ballast resistor that compensates for increased voltage by increasing resistance and therefore drops a greater voltage across itself.
The Barretter uses iron wire and in this device the current is stabilised at 300 mA. The voltage dropped by the tube is 95 to 165 volts.
The central glass rod supports the hooks from which the wire is draped.
The filament is a single strand of wire.
The word barretter is etched into the glass as is the Marconi name and the 304 Type designation.
The balloon envelope is 56 mm in diameter and, including the Edison Screw base is 126 mm tall.
References: Data-sheet & 1043. Type 304 was first introduced in 1935.