The ageing of gas-filled standard lamps on A.C. and D.C.

A comparison is made of the rates of fall of light output and current, at constant voltage, and colour temperature 2700? K approx., of 100-110 V 100 W gas-filled photometric standard lamps, on a.c. and d.c. supplies. The rates are appreciably greater on d.c. than on a.c., and this is shown to be due to specific effects of d.c.-operation on filament resistance, and filament emissivity. These effects are shown to be fully reversible, after many hours of burning, by reversal of the d.c. polarity, and not inconsistent with migration of tungsten along the surface of the filament. A practical conclusion is that the usefully smaller rate of depreciation characteristic of a.c.-operation may be fully retained on d.c. by reversal of polarity at regular convenient intervals. An a.c. supply, with its easy voltage regulation, is then legitimate for burning the lamps for the lengthy periods of the normal preliminary ageing tests.