diametrically opposed plates 4 which are simultaneously contacted by brushes 6 and 7 are electrically connected to each other via a capacitor 15 line 9 is also connected to ground 18 via a short circuiting capacitor 17 in the illustrated position brush 6 is short circuited with brush 7 for the alternating voltage signal via capacitor 15 and short circuited to ground 18 via short circuiting capacitor 17 in other words the complex resistance of d c motor 1 measured in this position tends toward zero this periodic significant change in the complex resistance which recurs twice during one revolution may be easily used to detect the rotation or to detect the rotational speed directly fig3 shows a circuit diagram for a rotation detection of this type a coil l 2 and a resistor r 6 are illustrated as an equivalent circuit diagram of the d c motor according to fig2 the inductance of coil l 2 and ohmic resistance r 6 represent the electrical properties of the winding branches capacitor 15 identified by c 5 in fig3 is shown which periodically bridges the winding branches of the d c motor this is illustrated schematically by switch u 2 a supply direct voltage is supplied to the d c motor itself via a direct power source 20 identified as v 2 over supply lines 9 inductance coils l 1 and l 3 which are otherwise.