Abstract: A probe assembly for measuring the speed of rotation of a gas-turbine engine compressor shaft has a sensor head mounted at one end of a tubular probe body by means of a bellows unit which permits limited axial displacement of the sensor head relative to the probe body. The sensor head has a sleeve sealed to its rear end, one end of the bellows unit being sealed to the sleeve and the other end being sealed to the probe body. The forward end of the probe body extends through the bellows unit and into the sleeve where it is resiliently coupled with the sensor head by a helical spring. Four outwardly inclined leaf springs are mounted on the end of the probe body within the sleeve and are resiliently urged against the inner surface of the sleeve so that they are free to slide over the surface of the sleeve while separation of the sensor head from the probe body is limited by engagement of the springs with an inwardly projecting lip on the sleeve.