A slab with one surface cut and etched parallel to (100) crystallographic planes formed the cathode in a low power electron microscope. The electron image of the cathode was formed on a fluorescent screen with a small hole at the center. An electron collector placed behind the hole permitted a measure of the electron current passing through the hole as a function of the temperature of the cathode as measured with an optical pyrometer.The most probable value of the work function, &phgr;, obtained after 2400 hours of outgassing in a sealed‐off, gettered tube is 4.59±0.02 ev. This may be compared with 4.56 ev obtained previously by Nichols for the work function in the [100] direction of a tungsten single crystal wire.