A rolling mill has a transducer for measuring the separation of a movable part of the mill from the fixed part of the mill. The transducer comprises a magnetostrictive element secured to one end of the parts, a transmitter for transmitting magnetostrictive signals into the element, a receiver for receiving signals reflected back along the element and magnetic means secured to the other part which produces a magnetic field which serves to reflect the signals along the element. Time measuring means measure the time interval between transmitting the energy and receiving the echo and this is representative of the distance between the two parts of the rolling mill.