Systems which employ two amplifiers—one to amplify, the other to integrate—for precise and/or sensitive determination of magnetic flux changes are described and their operating characteristics reported. One system measures flux changes in time intervals of 10−2 to 10−1 sec using moderately broad band amplifiers and gated circuits. This system discriminates against dc thermals with a gated self‐balancing potentiometer, and combines sensitivity and precision with the convenience of rapid readout on a digital voltmeter. The sensitivity of the system is 10−7 V sec, the accuracy better than one part in 104, and combined with pickup coils, permits detection of changes of 10−3 G. This system has been applied to measurements of saturation magnetization, Curie temperature, and susceptibility of iron and its dilute alloys. An alternative system uses a highly stable, dc chopper amplifier in which case the flux changes are produced in seconds and can be followed for minutes. This method has been essential for measurements of the change of magnetization of iron and nickel with pressure where many seconds are necessary to insure isothermal conditions. Sensitivity is the same as above. In another system for the measurement of the temperature dependence of susceptibility, a null coil is wound on the surface of a cylindrical sample which is oscillated near a pair of detector coils at 6 cps. The coil output is amplified, chopped in phase with the signal and integrated. This system has a sensitivity of Δχ = 10−10 and has been used to obtain the temperature dependence of the susceptibility of vanadium.