Abstract: An improved ultrasonic thermometer of the acoustic pulse-echo type for measuring high temperatures and temperature profiles having a magnetostrictive head surrounded by an acoustic pulse generating and pulse sensing apparatus having a positioned electromagnetic coil, the magnetostrictive head joined to an elongated heat sensing wire having a plurality of discontinuities at the distal end thereof to reflect a portion of the propagated acoustic pulse, the improvement comprising a lock fixedly holding the acoustic pulse generating and pulse sensing electromagnetic coil in alignment after initial adjustment in order to achieve long term stability.