proportional to the speed of rotation of the position transmitter the method described herein has the advantage that auxiliary voltage v z is composed of voltage waveform blocks of sum voltage v s and difference voltage v d which are in the relatively linear portion of an approximately sinusoidal waveform curve differentiated voltage v di is therefore composed of approximate square wave pulses each such square wave pulse having relatively little ripple if differentiated voltage v di is rectified a dc voltage having a relatively small ripple is obtained as an output voltage v a which is proportional to the speed of rotation of the speed transmitter in order that the polarity of output signal v a correspond to the direction of rotation of the position transmitter controlled rectification is required so as to produce a controlled combination of the positive and negative pulses of differentiator voltage v di this is achieved by using the inverted differentiated voltage v di for forming output voltage v a as long as the polarities of transmitter voltages v e1 and v e2 are equal and the noninverted differentiated voltage v di is used as long as the polarities of the two transmitter voltages are different from each other in the illustrative example of fig1 which describes a positive direction of rotation output signal v a is also positive fig2 shows a plurality of waveforms arranged on a common time scale the waveforms being similar to those described hereinabove with respect to fig1 except that the direction of rotation of the speed transmitter is negative in fig2 therefore transmitter voltage v e2 fig2 b does not lag transmitter voltage v e1 fig2 a by 90 u00b0.