The fleming valve detector consists of a carbon or tungsten filament lamp, in the bulb of which is also included a metal plate insulated from the filament, and the connecting wire of which is brought through the glass wall of the bulb to a third terminal outside. If the filament be rendered incandescent by the application of a suitable battery to its terminals the space between the filament and the insulated plate will be found to possess an unilateral conductivity, and if it be now included in a ciruit in which oscilations are taking place it will by its rectifying action convert them into an unidirectional current capable of actuating a telephone receiver.

The valve is shown in Fig. 47, which also shows the method of connecting it to the circuit.