Does anyone happen to have a lamp similar to this in their collection, or know of any reference that might be able to identify it? I am having some difficulty in finding out exactly what it is!
Some old English books show a line drawing of an identical lamp and label it as being an Osmium filament type, however I do not believe that. The type of cap is far too modern for that. Also it is marked SF Auer (presumably Societe Francaise), and osmium filament lamps were only ever made in Berlin and Vienna, not France.
I suspect it to be a more modern sintered tungsten or drawn wire tungsten filament lamp - but why the special mount construction?
The filament is mounted in a large diameter coil which spirals itself around the central ceramic column in 26 small segments. It appears to be all one continuous wire rather than 26 individual filaments. The supports are unusual. Heavy-gauge wires are clamped into the ceramic spine, and the ends of each support are formed into small hooks, each of which holds a short length of very fine wire of about the same diameter as the filament. The filament is attached to each of these fine support wires with a small metallic globule. It looks similar to how sintered tungsten filaments are arc-welded to their support wires - but that would be virtually impossible to weld together two such fine wires, at right angles to each other, without burning right through the filament wire. In Auer's patents on Osmium lamps he does describe a technique of mounting the filaments to the supports using an organic gum which bears the metal in solution - this would be painted onto the joint between the filament and its support and then heated, to decompose the joint to the pure osmium metal. The type of joint here looks similar to one made by that process, but I am not aware of such techniques being used before on tungsten filament lamps.
In all it is something of a mystery. If anyone can shed any light on it, I would be most grateful!