An investigation is described in which a high‐sensitivity analytical balance of standard design is used for determination of the surface density of that thorium layer which corresponds to maximum thermionic emission with the thorium‐on‐tungsten system. Following Brattain and Becker's definition of this value of surface density as one ``funit,'' thorium vapor is allowed to accumulate on a quartz plate until a weighable number offunits (about 600) has been deposited. The thorium flux infunits per minute is monitored periodically by means of a tungsten filament diode during the course of the deposition. Two polycrystalline specimens of ribbon from the same source have yielded the value of (4.2±0.2)1014atoms/cm2as the density for maximum emission. This is to be compared with a value of 5.0×1014atoms/cm2obtained in 1934 on theoretical grounds by Langmuir.