of rotation the wall portions 37 close the passageways 34 serving to drive the rotor 28 is a reversible direct current motor linked to the drive shaft 33 by means of a reduction gear and a flexible coupling to generate pressure pulse signals the direct current motor is powered by current of changing direction so that it periodically reverses its direction of rotation moving the rotor 28 alternately into the illustrated passing position and into the closing position offset by an angle of 45 u00b0 for example the respective end position of the rotor 28 is sensed by an angle of rotation transducer for control of the direct current motor in operation drilling fluid is conveyed through the drilling fluid conduit 2 of the drill collar 3 and the housing 4 of the borehole logging apparatus 1 in the manner illustrated in the figure by the arrowed lines with the drilling fluid current being produced by drilling fluid pumps connected to the drill string on the earth 39 s surface the drilling fluid current entering the housing conduit 9 at a pressure p 1 is throttled to a pressure p 2 lt p 1 as it passes the throttling conduit 22 the pressure differential p 1 u2212 p 2 becomes effective on the throttling section 16 and the measuring section 17 of the control piston 15 in the same direction and attempts to displace the control piston 15 in the direction of the signal transmitter 6 until the pressure.