The origin of the frequently observed long and short term current drifts after a change in the extractor voltage for the ZrO/W(100) Schottky cathode has been investigated. It has been found that a reversible, field dependent change in the equilibrium work function and shape of the cathode end form occurs at the typical operating temperature of 1800 K. The shape change results in three distinct geometric end form shapes. Although these end forms have been observed previously [L. W. Swanson and G. A. Schwind, in Handbook of Charged Particle Optics, 2nd ed., edited by J. Orloff (CRC, New York, 2008), Chap. 1, p. 1; S. Fujita, T. R. Wells, W. Ushio, H. Sato, and M. M. El-Gomati, J. Microsc. 235, 215 (2010); M. S. Bronsgeest and P. Kruit, J. Vac. Sci. Technol. B 27, 2524 (2009)], the purpose of this study is to elucidate the operating conditions under which these changes occur and the associated changes in emission properties. Emitters in the radii range 200-900 nm were investigated over the typical current density operating range employed in most electron probe forming systems. A correlation between work function and electric field has been found, which can be explained by a reversible, field induced change in the ZrO equilibrium coverage in the high field region of the emitter, thereby causing a change in the work function. It is believed that these field induced changes in the equilibrium work function and emitter shape at 1800 K explain the short and long term drifts observed after a change in extractor voltage. (c) 2010 American Vacuum Society. [DOI: 10.1116/1.3501362]