The heater will take a defined time to heat the bi-metallic strip to the point that the contacts will make. Such switches were used to delay the HT voltages until the heaters of the valves were at the correct temperatures. Mercury vapour rectifiers and travelling wave tubes are examples of devices that are protected by thermal delay switches.
The heater is enclosed in a ceramic tube to provide the delayed warm-up.
The lettering shows the Ediswan brand and Made in England.
Thanks to Mark Mallison for sending the images.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 45 mm tall.
Reference: Data-sheet. Type DLS19 was first introduced in 1946. See also1946 adverts.
