This paper describes an experimental investigation of the dependence of the space-charge smoothing factor on valve geometry. It is found that ifAis expressed as a function of the ratio between barrier-to-anode potential and mean emission energy (expressed in volts),Ais independent of valve geometry.The results are compared with several hypotheses and the discrepancies noted. The various hypotheses, and the relations between them, are discussed in some detail.A spurious effect, of unknown origin, appears to enter whenA≃ 0.1, and further experiments are suggested.