Lamp load

In case of a tungsten filament lamp, the filament resistance when the lamp is switched off (cold filament) is 10 times smaller than in case of the lamp switched on (hot filament).
After the Reed contact commutation, at after the lamp turning on, for a short time the in-rush current is 10 times higher than the one circulating in steady state. This flow may damage the Reed contact or jeopardize its duration. In this case, the solution is to introduce a resistance in series to the Reed switch, this cutting the maximum value of the current, or a resistance in parallel to the Reed switch, to keep the filament hot (by increasing the resistance) without causing the lamp turning on.