Electrical measurements with special reference to lamp testing

Perhaps there is no kind of testing for commercial results that requires more accurate electrical measurements than the testing of incandescent lamps. All incandescent lamps are very sensitive to changes in the electrical conditions of the circuit. A change of 1 per cent in pressure brings about a change of 5.7 per cent in luminous intensity for carbon lamps and 3.7 per cent for tungsten filament lamps. There is a corresponding change in the life, but of a magnitude four to five times as great.