I have one of these lamps, it came from another collector in your neck of the woods (Fin Stewart). It was described as a resistance lamp and indeed the construction would lend itsself more to such a use. The hefty carbon filament is definately not made for lighting. However I do believe, as Tim has mentioned, that its primary purpose was a heater bulb. If it were intended as a resistance lamp, it would be logical to me that its resistance be printed on the glass as is the case with all the other resistance lamps I have seen so far. However it has a voltage and wattage rating implying it is meant for direct connection to the mains supply. I havn`t tried this with my lamp as I don`t know what would happen (or have a suitable fixture for it either), but suspect it would burn very hot and rather orangey, like the more common single ended carbon heater lamps.
I`m not exactly sure of the purpose of the darkened end but its construction looks similar to that found in some discharge lamps, with the disc shaped electrodes adjacent to where the filament connects to the lead-in wireat that end. The blackened coating is, I believe, a Rhodium (or similar) getter intended to absorb impuritie, and the lamp would probably have been burned with that end at the top. I suspect this was employed to keep the glass clean and clear during the life, otherwise the characteristic blackening from vaporised carbon would significantly impede the rediated heat as the lamp ages.
I don`t know any of this for sure, but it makes the most sense to me.
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