PURPOSE: To realize a direct reading for the speed of current, by detecting the vibrating fluid caused by heating with two pieces of detecting elements and then selecting the heating frequency so that the phase always satisfy a specific relation.

CONSTITUTION: The heating wire 3 is heated by the output obtained by amplifying 9 the output of the frequency variable oscillator 6, and the frequency at this moment is monitored by the counter 7. The heat current flows down by the heating of wire 3, and this heat current vibration is detected by the detecting elements 4 and 5. The outputs obtained at the heat detectors 10 and 11 are applied to the X and Y axes of synchroscope 12. Then the frequency of oscillator 6 is controlled to secure a phase difference of 180° for the displayed waveform, and the speed of current is read directly from the frequency of counter 7 at the moment when the phase difference becomes 180°.