He called his instrument a vibrating reed magnetometer; it was only later that AGFM became the preferred name. Zijlstra designed his instrument in order to study the hysteresis curves of microscopic (a few μm across) single domain particles. Using a fine (38 μm x 20 mm) gold wire as a resonant element, a resolution of 10-11 J T-1 was reported. Zijlstra’s experimental set-up was configured so that the alternating gradient field was oriented vertically, in the same direction as the applied DC field. The resonant element was mounted perpendicular to the field (i.e. horizontally). The displacement of the reed was measured visually, with the aid of a calibrated microscope, and a stroboscopic lamp to ‘freeze’ the motion of the fibre at its maximum deflection.