The operation of any valve type depends on the current flow caused by drawing free electrons from an emissive cathode to the anode by a positive potential being applied to the anode.  By applying a potential to grids interposed between the anode and cathode of a valve, the flow of electrons may be controlled - vary the grid voltage and a corresponding variation in anode current results which may be greater than the voltage applied to the grid and hence this is how a valve ampifies a signal.

However, rather than using grid potential to produce an electrical variation, we sometimes need to produce electrical variations which correspond to variations in light and this is where the photocell comes in.    To understand the photocell we have to do a little chemistry, as we know, some elements are very stable, however others are less so.  In Caesium, the energy from light can upset a Caesium atom's stability and force it to give up electrons.   However, 'dope' Caesium with Silver and Antimony to form an amalgam and an element of stability is introduced whereby only specific quantities of electrons are produced in reaction to a specific level of light and it's colour and it is on this basic premise that a photocell works  Right, that's enough chemistry for today so for anyone still awake, I shall just let you know that in my next blog entry we will look at types of photo-electric devices in a little more detail