The inductance of a solenoid depends on the properties of its core. Adding a ferromagnetic material will raise it, and a diamagnetic material will do the opposite.

The Inductance meter on the device simply pairs the externally plugged in inductor with a high stability capacitor on the inside. The resultant tank circuit is driven at its resonant frequency which is measured by the frequency counter. Frequency of oscillations is given byWhere the inductance L, depends on the solenoid, and its core. C is 1000pF.

The video shows the frequency counter with a resolution of .2Hz connected to the L meter's output logging variations in a solenoid's inductance. Inserting a ferromagnetic material changes the frequency( A key in this case gives up to 6000 points after being pushed 1cm into the core . After calibrating, and accounting for noise, the resolution is less than 10 microns )

This method has tremendous applications in mechanics labs. However , It won't match up to optical methods when it comes to being unobtrusive. The oscillating field of the solenoid should cause a minor perturbation in the ferromagnetic piece attached to the body being recorded.