A sonic strain pulse is induced in a specially designed magnetostrictive waveguide by the momentary interaction of two magnetic fields.

One field comes from a movable permanent magnet which passes along the outside of the sensor tube, the other field comes from a current pulse or interrogation pulse applied along the waveguide.

The interaction of the two magnetic fields produces a strain pulse, which travels at sonic speed along the waveguide until the pulse is detected at the head of the sensor.

The position of the magnet is determined with high precision by measuring the elapsed time between the application of the interrogation pulse and the arrival of the resulting strain pulse.