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This exhibit is a thermal delay switch, used to delay the application of anode volts until associated valves had time for the cathodes/filaments to reach operating temperature.

This looks like one of the peculiar little Ediswan devices used in 'Metrovick' AC Mains Adapter units marketed around 1930 to enable owners of domestic battery radios to operate their existing sets from the (newly installed) AC mains which was at that time spreading rapidly.

In addition to a multi-winding mains transformer, these Adapter units included an HT rectifier giving around 100 to 120V DC, and a gas-filled (argon?) LT rectifier.

When 'ON' the radio set operated from its own LT accumulator but with HT supplied by the Adapter; when 'OFF', the Adapter trickle-charged the LT accumulator by means of the LT rectifier. A device similar to this exhibit was fitted into the Adapter (black base cap upwards) and may have been a delay switch for the LT rectifier.

The coiled filament is clearly visible as is the bi-metallic strip.

The wide glass tube envelope is 17 mm in diameter and, excluding the base pins, is 68 mm tall.

References: Observation & private communication.