Today, we are going to talk about photocells and in particular 'gas amplification,' an interesting phenomenon shown by gas filled photocells and I was reminded of a hilarious occurrence when a vintage radio 'expert/forum gobs__t' proudly visited proclaiming that he had just 'stolen' a Baird Televisor lamp and on proudly producing it with a smug flourish, I was able to deflate his arch sense of triumph by regrettably informing him that in actuality he had 'stolen' a Mullard 56CG photocell.
With photocells, although the manufacturing methodology is the same as for a standard thermionic valve, a precise aliquot of a noble (inert) gas was introduced after a vacuuum had been pulled from the envelope. The low vapour pressure of the injected gas means that it's molecules are distributed within the envelope and indeed the electrode cage. In operation, a portion of cathode electron emission will collide with the inert gas molecules resulting in ionisation. The secondary emission electrons so produced would then be attracted to the anode and conversely, the gaseous cations would be attracted to the cathode where they 'belt it' so hard that even more cathode emission occurs. The effect of this cascade reaction is that anode current would be greater than with an equivalent vacuum cell by up to a factor of 10X! And this my friends, is 'gas amplification.'
Gas filled photocells are very sensitive to low light levels and can discriminate very small changes in light as may be seen with sound-on-film equipment. Mullard produced a whole range of this type of photocell and so help you recognise them should you come across examples, I present below a photograph of the range, aren't they sweet: -