A Rankine balance type magnetic susceptometer of much greater sensitivity than previously obtained and combined for the first time with a flow system for the study of chemical reactions is described. The instrument can detect in one measurement a change in volume magnetic susceptibility of 5×10−12emu. The response time is adjustable down to a fraction of a second by means of an electronic servo system. The time resolution of the flow system is presently limited by the flushing time of the cell to five seconds. A time resolution of hundredths of a second is feasible, as is the detection, in one measurement taking a second, of a change of 1.5×10−12emu corresponding to the rms Brownian force arising from air resistance to the motion of the magnet.