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This exhibit is clearly a thermal switch. A thick wire helical heater is supported horizontally with a bi-metallic strip curved round it with a rod contact at the tip. A horizontal wire forms the other contact. Both contacts are notched.
On test one Volt across the filament causes a 500 mA current to flow that quickly heats the moving contact that bends and opens away from the fixed contact. The filament does not show any colour change. A higher current will cause the filament to glow orange and the moving contact to open considerably from the static rod.
Normally thermal delay switches are used to prevent HT being applied to a mercury vapour rectifier until it has fully heated - around 30 seconds. This device offers no such delay.
The moving contact has a twist in the bi-metallic strip.
The coil passing across the strip. The stud contacts looked to have been wiped from left to right when solidifying but the base has a key that indicates the socket it was designed for required this switch to be pushed in. Given the absence of vertical scratches on the studs, one can conclude that this device has not been used. Also there is no pitting on the contacts within the glass.
The base has a grip ring, a diamond to show orientation of the key and an embossed S. Do the crosses indicate a faulty product?
The thin glass tube envelope is 18 mm in diameter, and excluding the base is 72 mm tall.
Reference: Observation.

 

Updated August 23, 2017.
shape:thin.glass.tube construction:pinch base: pins:4 pin:
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