Abstract: A core of magnetic material in the shape of a closed loop has three windings thereon. The first winding forms a circuit with a lamp to be monitored. As long as the lamp's filament is intact, whether or not the lamp is energized, this circuit can conduct current. The second winding is connected to a pulse generator to induce variations in the magnetic flux in the core. The third winding detects these variations in flux and a detection circuit connected to the third winding produces an indication if the variations are greater than a predetermined threshold. If the lamp is energized, the current in the first winding causes the core to be saturated with magnetic flux and thus the induced flux variations are below the detection threshold. If the lamp is unenergized, but intact, the current pulses in the second winding induce counter-pulses in the first winding which counteract the effect of the pulses in the second winding and thus the induced flux changes are below the detection threshold.