The vacuum requirements to obtain stable field emission cathodes are discussed. To keep the average work function of a tungsten emitter constant, the total pressure of all chemically active gases should not exceed 10−14Torr. To minimize ion bombardment of the emitter surface the helium pressure should be reduced by ion pumping even with tubes made of an alumino‐silicate glass. The relationship between the current density and the slope of a Fowler‐Nordheim plot has been calculated. Experimental evidence is given that variations in the work function can be compensated by changes in the field strength in such a way that stable field emitters are obtained. A qualitative explanation of such a mechanism is suggested.