Abstract: An automatic drive circuit for an ultrasonic probe, such as a dental scaler insert, is operative with a probe which includes a magnetostrictive unit and a vibratory element to be set into vibration by the magnetostrictive unit upon energization of the magnetostrictive unit with an alternating magnetic field. An energizing coil is enclosed within a handle having a compartment for receiving the probe. The coil is located within the handle adjacent to the probe-receiving compartment for applying the alternating magnetic field to the magnetostrictive unit upon insertion of the probe into the handle and upon energizing the coil by the drive circuit. An oscillator of the drive circuit is coupled to the coil for applying an oscillatory current to the coil. The drive circuit includes a frequency sensor for sensing a frequency of the oscillatory current, the sensor outputting a signal designating a magnitude of the frequency.