A phototube consists principally of two electrodes in an evacuated glass envelope.
One electrode, the cathode, emits electrons when its sensitized surface is exposed to light or other radiant energy.
These electrons are drawn to the second electrode, the anode, because this electrode is operated at a positive potential.
The number of electrons emitted by the cathode depends on the wave length and the amount of radiant energy falling on it.
The phototube thus provides an electric current whose magnitude can be controlled by light or other radiant energy.