This paper is concerned with the double sheath which is normally present in front of a hot cathode in a low‐pressure dc discharge. The authors present a theory of an idealized form of this sheath, under space‐charge‐limited cathode emission conditions. The discrepancies from this model to be expected in a practical situation are discussed, and comparisons are made with experimental results. To obtain these, an electron beam probing technique was used, and electric field measurements were made directly in the cathode sheath of a low‐pressure mercury‐vapor discharge. Good agreement was obtained between the simplified theory and the experiments.