A description of cold cathode magnetrons operating as gauges has been developed in terms of the conditions for establishing a discharge and the effects of space-charge buildup. Pressure response curves for NRC type 552 (Redhead) and GCA model 1410 (Kreisman) gauges were determined by a conductance-regulated, pressure-ratio method using Bayard–Alpert type gauges for upstream pressure reference. Four characteristics of normal magnetrons operated below cutoff are predicted from the discharge mechanism and correlated with test data: (1) an out-of-strike state characterized by a steady pressure-independent gauge reading (about5×10−14Torr) due to leakage current; (2) a minimum pressure threshold for operation,2.7×10−12Torr for the NRC 552,1.7×10−10Torr for the GCA 1410; (3) a range (below10−9Torr) of nonlinear pressure-dependent response moderated by space charge buildup; and (4) a range of near linear response corresponding to the region of nearly saturated space charge.