When I bought this funny looking lamp from a junk dealer at Wimborne Market I thought it was a Miners' Lamp but I've now discovered it was used in sewers to detect gas. I don't know why this is needed as I imagined that all sewers are filled with gas anyway but presumably it's used as a means of letting the chaps that visit such places that the level of gas is too high for safety?
The Spiralarm has the maker's name J.H.NAYLOR, WIGAN and has Type "S" marked on the label. I understand the Type "M" was used in mines and has a slightly different characteristic so that the gas type and percentage that it detects is different.
The patent number is given on the label as 352267.
reveals the patent details, from which can be seen that the design dates to 1931. I should think that my model is rather later than that, but I shall leave that to experts in the matter.
Operation is relatively simple. In the presence of an inflammable gas in the surrounding air the flame gets bigger and heats a bimetal strip which operates a set of contacts, turning on a lamp in the lower compartment.
The materials from which the lamp is chiefly constructed are aluminium (or one of its more robust alloys), and brass.
As you can see a spare wick is looped around the base.. in fact it's possible that the lamp has never been used and the original? wick never fitted.
Clipped onto the base is a mirror made from chrome plated steel.
A knurled brass adjuster is located underneath the base. This presumably to enable the flame height to be preset and to allow for wick to be raised when it's burned down.