This instrument was designed to measure and control the flux density between the poles of an iron-core electromagnet used in conjunction with a beta-ray spectrometer. A sensing coil, rotating at 1800 rpm, generates 0.115 v (rms) per gauss. A comparison signal, against which the emf of the coil is bucked, is generated electrostatically by a dynamic condenser. Its rotor plates are shaped to produce a sinusoidal wave form. The amplitude of the comparison signal is determined by a dc voltage applied between two stator sections which are juxtaposed on opposite sides of the rotor shaft. This voltage thus provides a measure of the field strength. An unbalanced condition results in the appearance of a 30-cps error signal at the stator sections (which are isolated with respect to ac). After amplification and synchronous rectification, this signal indicates the degree and phase of the imbalance. The motor speed is maintained constant by a precision 60-cps tuning fork. The stability of the fluxmeter is of the order of one part in 2500, as determined by the reproducibility of the positions of internal-conversion-electron lines from Bi207 and Cs137 sources.