A special electron-optical system for producing a low-energy beam, as far as possible only in the absence of a magnetic field, is described. Its use for measuring both the strength of d.c. magnetic fields relative to a static reference field, and the relative strengths of time-dependent magnetic fields, with the aid of the voltage pulse which it produces at field zero, is described. The various factors affecting the amplitude and width of such pulses are discussed, and quasi-static measurements are given in support of the conclusions derived from a simple theoretical analysis of such factors. The operation of such an instrument in timedependent magnetic fields, such as those found in particle accelerators like the betatron and electron-synchrotron, is briefly discussed.