fig1 shows a shaft 1 , which is driven at an approximately constant speed by a motor 1a . a stroboscopic disc 2 is fixed to the shaft 1 , which at one end forms a crank 3 , which extends through a slot 4 in a lever 5 . by said lever 5 , an oscillating pivotal movement about a horizontal axis 7 is imparted to a sound transducer head 6 , which comprises a sound transducer . when the angular movement of the shaft 1 is designated ( a ), the angular movement of the sound transducer head 6 is designated ( b ), the radius of the crank is designated ( r ) and the distance between the pivotal axis 7 and the axis of the shaft 1 is designated ( d ), the following relationship exists : ## equ1 ## other relationships will be obtained if the slot 4 is not straight but curved , e . g ., in the form of a curved cam slot . besides , the simple crank drive 1 , 3 may be replaced by a plurality of links . in any case , there is a definite functional relationship between the angular movement ( b ) of the sound transducer head 6 and the angular movement ( a ) of the shaft 1 so that the general statements can be made . similar relationships will be obtained if the crank drive 1 , 3 is used to reciprocate a slide which carries the sound transducer head so that the movement ( b ) is a linear movement . it has been mentioned before that the stroboscopic disc 2 is secured to the motor shaft 1 . throughout its periphery , the disc 2 is provided with regularly spaced , light - transmitting slots 8 . a light source 9 and a light sensor 10 , such as a photodiode , are disposed on opposite sides of the disc 2 adjacent to the slots 8 . the disc 2 carries also a control element 11 , such as a magnet , which opens for a short time a normally closed resetting switch 12 once during each revolution of the disc 2 . the switch 12 could be alternatively actuated by different means , which may be electrical , photoelectric or mechanical . the pitch of the slots 8 could be varied so that pulses are projected by the sound transducer head 6 with pulse repetition periods corresponding to equal increments of angular movement in response to the output pulses of the photodiode 10 . the stroboscopic disc 2 may be so large that it permits a generation of an adequate number of primary pulses . in the embodiment shown by way of example , the slots 8 are spaced regularly and are much larger in number than the sound pulses to be projected during a complete scanning cycle of the reciprocating sound transducer head . it is apparent from fig2 that primary pulses generated by the primary pulse generator 8 , 9 , 10 are applied to a counter 13 , which advances one step in response to each primary pulse and returns to its initial position when the switch 12 is opened . each count of the counter 13 designates an address of a memory 14 , which in the illustrated embodiment consists of a prom , which at each storage location stores either a ` 0 ` or a ` 1 ` in accordance with a preselected program . the memory 14 is connected by a lead 15 to the pulse generator and receiver 24 of the ultrasonic examination and display equipment and is connected by a lead 16 to another counter 17 . if a ` 0 ` is stored in the memory 14 at the storage location selected by the counter 13 , no signal will appear in leads 15 , 16 . if a ` 1 ` is stored at the selected location , an output signal will be delivered via lead 15 to activate the pulse generator 24 so that a sound pulse is projected by the sound transducer head 6 , and the counter 17 is advanced one step . the counter 17 constitutes the address selector for another memory 18 , which consists also of a prom . the memory 14 is a scanning pulse control memory and the memory 17 a deflection control memory , in which one or more values determining the deflection of the base line of a fluorescent screen of the ultrasonic examination and display equipment is or are stored at each storage location . when a given storage location of the memory 18 has been addressed the value or values stored therein are fed via a lead 19 to the deflection control unit 25 . in response to said value or values and to the sawtooth voltage applied to the control unit 25 via a lead that is not shown , the control unit 25 generates the voltages required to deflect the base line on the fluorescent screen 26 . these voltages are delivered via lead 27 . in response to the opening of the switch 12 , the counter 17 is also reset to its initial position . echoes generated in response to the sound pulses projected by the sound transducer head 6 are received by the latter and converted into echo signals , which are delivered to the pulse generator and receiver 24 and are amplified and shaped in a processing unit 28 and then delivered via a lead 29 and used to unblank the tracing ray . the dotted line 30 indicates that the vertical deflection may alternatively be controlled via the lead 15 . it has already been mentioned that the number of slots 8 greatly exceeds the number of sound pulses to be projected during each scanning cycle of the pivotally moving sound transducer head . as a result , the pulse repetition period of the projected sound pulses can be varied in steps so that the section surface is scanned with a transverse resolution which is uniform to a fairly close approximation . any disturbing signals will not appreciably affect the accuracy of the display . when it is assumed that the number of slots is ten times the number of sound pulses required , a disturbing pulse would cause the sound pulse to be triggered too early by a time which corresponds to 10 % of the sound pulse pitch . if the transverse resolution amounts to 0 . 5 °, the actual error will amount to 0 . 05 °. because of the resetting effected by the switch 12 , that error will arise only during the scan in which the disturbing pulse occured . a selection of the transverse resolution with which the section surface is to be scanned and an adaptation of the equipment to different sound transducer heads or scanning ranges may be effected if the memories 14 and 18 are replaced by memories having different programs for the scanning control and the deflection control . alternatively , the memories which are used may have a much larger storage capacity than is required for one scanning control program or deflection control program so that a plurality of scanning control programs or deflection control programs can be stored at different storage locations . in that case , selector switches may be provided for effecting a change to another scanning control program and another deflection control program . alternatively , such change can be effected in that adders are associated with the counters 13 and 17 and in response to a change of the program add a certain number to the count of the respective counter so that the correct address is obtained . fig3 is a sectional view showing the scanning unit of fig1 . in order to avoid an influence of extraneous light on the photodiode 10 , the latter is preceded by a stop plate 20 , which has a hole 21 . one of the slots 8 of the disc 2 may become clogged so that a primary pulse is skipped and an error results . in accordance with fig4 this is avoided in that the lamp 4 is succeeded by a lens 22 , which spreads the emitted light in parallel rays over a plurality of slots . the stop plate 20a has a corresponding number of holes 21a and thus constitutes an array of passages , which is succeeded by another lens 23 , which focuses the light onto the photodiode . even when one slot of the disc 8 is clogged in that arrangement , a primary pulse , which in that case has a somewhat smaller amplitude , will be generated in response to the movement of the disc 2 relative to the hole arrays 20a , 21a . the memories 14 and 18 are programmed by the manufacturer of the equipment by means of a programming device . instead of the proms , normal memories or rams may be used if the equipment includes microprocessors in which the scanning control program and the deflection control program are automatically computed , whereafter the programs are stored in the respective memories . in computing such programs , these microprocessors will utilize input parameters which include the functional relationship between the angular movement of the drive shaft and the scanning movement of the sound beam as well as the number of sound pulses desired per scan . so far , only the function of one detector i has been described with reference to fig4 . the embodiment shown by way of example comprises two detectors i and ii , and the detector ii consists of the same elements as the detector i . in the detector ii , these elements are designated 9 &# 39 ;, 10 &# 39 ; etc . the two detectors i and ii comprising the stop plates 20a and 20a &# 39 ; are arranged relative to the disc 2 with a phase displacement so that the holes 21a and 21 &# 39 ; are in perfect register with the slots 8 of the disc 2 in different angular positions of the latter . as a result , the signals generated by the photodiodes 10 , 10 &# 39 ; are displaced in phase . these signals are fed to a processing device 31 , in which they are subjected to an electronic processing that is equivalent to a subdivision of the slot pitch . the output signals of the processing device 31 are applied to the counter 13 . for length measurement it is known to provide circuits in which phase - displaced signals resulting from a reading of a scale , particularly of an incremental scale , are subjected to an electronic multiplication corresponding to subdivision of the existing scale . phase multiplier circuits and potentiometer circuits have been used for the electronic multiplication . such circuits have been described , e . g ., in german patent publication no . 1 , 231 , 911 and the u . s . pat . nos . 3 , 496 , 374 ; 3 , 371 , 335 ; and 3 , 671 , 750 . if a suitable circuit is employed in the processing device 31 , the same may be used to generate a plurality of length - measuring signals per scanned hole and the offset detector i , ii in conjunction with the circuit 31 may be used to detect the direction of movement if the disc 2 or a corresponding scale is oscillated rather than moved always in one direction .