On measurement of a component Ay of the amplitude of the oscillations of the tip of a capillary which is clamped to a horn to which ultrasonics can be applied from an ultrasonic transducer, whereby measurement is based on the shading of a light beam by the capillary and whereby the intensity of the light beam is measured by means of an opto-receiver, the capillary is placed at one side of the light beam without it shading the light beam and ultrasonics is applied to it. The capillary is then moved in steps through the light beam whereby, for each of the i=1 to n steps, the direct voltage portion UDC(yi) and the alternating voltage portion UAC(yi) of the output signal of the opto-receiver are determined as well as a co-ordinate yi whereby the co-ordinate yi designates the position of the capillary in relation to a co-ordinate axis y running vertically to the light beam. The amplitude Ay is then calculated from the values measured. |