Large tungsten filament lamp; as it heats, current drops.

Large tungsten filament lamp, as it heats, current drops. This demo is similar to D+55+2 and D+55+8. When the knife switch is closed, 6 V.A.C. is put on a locomotive headlight which has a tungsten filament. At room temperature, the resistance of the filament is low and a large current will flow, initially pegging the needle of the projection ammeter. The filament rapidly heats up and the filament resistance rises. The current drops quickly to 15 amps and then slowly down to 13 amps.