In normal operation of receiving-type valves, with soda glass envelopes, a small current may flow through the glass to an external coating. Part of this current consists of positive ions generated by the flow of electrons through the atmosphere inside the envelope. As the pressure inside the valve falls during operation, the value of the ratio of the current through the envelope to the anode current flowing in the valve also falls, eventually reaching a stable value which is independent of pressure. This residual value is due to the emission of photoelectrons from the inside of the glass, when bombarded by X-rays generated at the anode. It therefore depends upon the nature of the anode surface and upon the anode potential.The continuous passage of this photoelectric current through the glass may generate small quantities of gas by electrolysis, and so affect the behaviour of the valve. Experimental evidence suggests that the risk of permanent damage to the cathode emission is very small.