When performing various jobs around the house was sometimes necessary to have on hand a source of electricity with lower than the normal network voltage. For example, to illuminate the water in the tank (to save the bulb from premature burnout of the filament), etc.
Help out in all these cases, a convenient “step-down” extension cord with two sockets, one of which is connected through the diode: it is “cut off” every negative half-wave, leaving only half the previous voltage.