The magnetic field of a permanent magnet has the effect of locally deforming the wave guide. The short current pulse through the copper conductor causes a magnetic field.
This magnetic field interferes with the existing field of the permanent magnet (position element). In result, an ultrasonic wave is generated. The sonic wave travels along the wave guide until it is detected by the pickup. The time between the start of the pulse and receiving of the receiver impulse is measured. The elapsed time indicated by the timer then represents the distance between the position magnet and the pickup.
The abbreviation BTL stands for Balluff Transsonar Linear Displacement Transceiver. Basically, the principle if "magnetostriction" is applied. Magnetostriction is also known by the term "joule effect". The effect is also responsible for the low-pitched humming sound that can be heard coming from transformers, where oscillating AC currents produce a changing magnetic field.