1. A method of measuring ultrasonically the thickness of an elongated member in which an on signal and an off signal are developed in response to the receipt of first and second associated echo pulses of predetermined test characteristics from one surface and an opposite surface, respectively, of the member and in which relative motion is caused to occur between the successive sections of the elongated member and a source of ultrasonic pulses, which includes the steps of: establishing a test period during which echo pulses are tested; transmitting an ultrasonic pulse into the member and then receiving a first echo pulse reflected from the one surface of the member and then receiving an associated second echo pulse from an opposite surface of the member; while excluding extraneous low level signals; testing the first echo pulse and then an associated second echo pulse for a qualifying predetermined minimum amplitude; initiating in response to receiving a qualifying first echo pulse the development of the on signal and the examination of a validating duration characteristic of the first echo pulse; conditioning partially in response to the first echo pulse being validated facilities for enabling the generation of a control pulse; initiating in response to the validation of the first echo pulse and the receipt of a qualifying associated second echo pulse and examination of validating test characteristics of a qualifying associated second echo pulse; while causing the development of the off signal with the time duration between the on signal and the off signal being related to the elapsed time between the receipt of the first echo pulse from the one surface and the receipt of the associated second echo pulse from the opposite surface; measuring the elapsed time between the on signal and the off signal; completing the conditioning of the enabling facilities in response to validating the associated second echo pulse to generate the control pulse; and converting the measured elapsed time in response to the generation of the control pulse into a signal the magnitude of which is proportional to the thickness of the elongated member.

5. A method of measuring ultrasonically the thickness of successive sections of an elongated member in which an on signal and an off signal are developed in response to the receipt of first and second associated echo pulses of predetermined test characteristics from one surface and an opposite surface, respectively, of the member and in which relative motion is caused to occur between the successive sections of the elongated member and a source of ultrasonic pulses, which includes the steps of: establishing a test period during which echo pulses are tested; transmitting an ultrasonic pulse into the member and then receiving a first echo pulse reflected from the one surface of the member and an associated second echo pulse from an opposite surface of the member; while excluding extraneous low level signals; testing the first echo pulse and then an associated second echo pulse for a qualifying predetermined minimum amplitude; initiating in response to receiving a qualifying first echo pulse the development of the on signal and the examination of a validating duration characteristic of the first echo pulse; conditioning partially in response to the first echo pulse being validated facilities for enabling the generation of a control pulse, a valid first pulse being one in which the predetermined minimum amplitude occurs only during a preset time; initiating in response to the validation of the first echo pulse and the receipt of a qualifying associated second echo pulse the examination of a validating duration characteristic of a qualifying associated second echo pulse; while causing the development of the off signal with the time duration between the on signal and the off signal being related to the elapsed time between the receipt of the first echo pulse from the one surface and the receipt of the associated second echo pulse from the opposite surface; measuring the elapsed time between the on signal and the off signal; completing the conditioning of the enabling facilities in response to validating the associated second echo pulse to generate the control pulse, a valid second echo pulse being one in which the minimum amplitude occurs within a preset duration; and determining the thickness of the elongated member by converting the measured elapsed time in response to the generation of the control pulse into a signal indicative of the thickness of the elongated member.

10. A method of controlling the operation of an apparatus for measuring the thickness of successive sections of a cable jacket, in which relative motion is being caused between the apparatus and the cable, and in which signal pulses are received with a predetermined sequence of pulses of predetermined test characteristics capable of causing the development of signals having a relationship therebetween which is related to a parameter being measured, which includes the steps of: establishing a time interval during which signal pulses are tested; while enabling the comparison of a first pulse received from an outwardly facing surface of the cable jacket and then an associated second pulse from an opposite inwardly facing surface of the cable jacket with respect to reference amplitude levels; developing automatically an on signal upon receipt of a first pulse of a predetermined minimum amplitude from the one surface of the jacket; testing the duration of the first pulse at the minimum amplitude; developing an off signal upon the validation of the first echo pulse and the receipt of an associated second echo pulse of a predetermined minimum amplitude from the opposite surface of the jacket with the second pulse occurring a predetermined time interval following the beginning of the first pulse; the time duration between the on signal and the off signal being related to the elapsed time between the receipt of the first pulse from the one surface of the jacket and the receipt of the associated second pulse from the opposite surface of the jacket; measuring the elapsed time between the on signal and the off signal; the amplitude of the first pulse being less than the minimum amplitude after a preset time being a partial prerequisite to enabling a generator for generating a pulse to control the disposition of the measured elapsed time; examining an associated second pulse for peak amplitude and a duration characteristic in response to validating a first pulse; the verification of duration of the second pulse being a second partial prerequisite for enabling the pulse generator; the verification of the peak amplitude of the associated second echo pulse completing the enabling of the pulse generator; initiating a cycle of operation while conditioning the circuit for another cycle of operation, the end of each test cycle operating the pulse generator if previously enabled; and converting the elapsed time into a measurement indicative of the thickness of the jacket.

11. A method of discriminating among pulses received from a sensor to indicate the receipt of a sequence of valid associated pulses, which includes the steps of: permitting after a predetermined delay the testing of pulses within a predetermined time interval; enabling the testing of one parameter of a first pulse upon the receipt thereof; developing an on signal upon receipt of a first pulse having a valid one test parameter; while further testing another parameter of the first pulse; the validation of the one parameter and then the other parameter of the first pulse being indicative of a valid first pulse; then developing an off signal in response to the validation of the first pulse and the receipt of an associated second pulse; and then measuring a relationship between the on signal and the off signal; enabling the testing of one parameter of a second pulse; while further testing the second pulse for the validity thereof with respect to another parameter; the validation of the first pulse and the validity of the second pulse with respect to the one parameter enabling a control pulse generator; the validation of the second pulse with respect to the other parameter further enabling the generator; the enabled generator being operated at the end of the predetermined time interval for causing the measured relationship tO be converted into the measurement of another parameter as an indication of the receipt of a sequence of valid associated first and second pulses.

12. A method of testing echo pulses received from surfaces of an elongated material being advanced under test in response to the pulsing of a transducer aligned with the material in order to measure the material, which includes the steps of: delaying for a predetermined time after the pulsing of the transducer the examination of echo pulses; enabling facilities for testing the threshold minimum amplitude of the first pulse received from one of the surfaces of the material under test while establishing a time interval during which echo pulses received are tested; testing the threshold minimum amplitude of a first echo pulse; impressing a signal on a device to enable the counting of pulses corresponding to the receipt of a first echo pulse; causing the device to begin counting; determining whether after a preset delay time the first echo pulse has decayed below the minimum amplitude, a valid first echo having the threshold minimum amplitude for less than the preset delay time; testing the threshold minimum amplitude of an associated second echo pulse for a qualifying value; discontinuing the count upon the receipt of a qualifying associated second echo pulse during the time interval; determining that the second echo pulse exceeds the qualifying value within a duration having a preset value; and testing the peak amplitude of the second echo pulse to determine if the peak amplitude is greater than a predetermined magnitude, a valid second pulse having a peak amplitude greater than the predetermined magnitude; the validity of the first echo pulse and the second echo pulse causing the count to be accepted and stored as a valid measurement.

13. A method of determining the validity of each successive sequence of associated first and second echo pulses reflected from surfaces subjected to ultrasonic waves, a valid first pulse having a predetermined minimum amplitude for a duration no greater than a preset value and a valid second pulse having a predetermined minimum amplitude and a duration greater than a preset time and of generating a control pulse for valid sequences of associated echo pulses, which includes the steps of: applying the first echo pulse and then the associated second echo pulse as inputs to a logic circuit; testing the minimum amplitude level of the first echo pulses and then the second echo pulse compared to the validity level; developing an on signal for a counter upon determining that the first echo pulse has the required level; causing a first echo pulse having the predetermined minimum amplitude to generate a control pulse to delay the examination of the existence of the first echo pulse after the preset value; determining whether a first echo pulse of the predetermined minimum amplitude is present after the preset value; developing an off signal for the counter upon validating the first echo pulse and receiving the second echo pulse; enabling a verification circuit for an associated second echo pulse subsequent to validating a first echo pulse; causing the enabled second echo pulse verification circuit to generate a holdoff and a validation pulse during the receipt of a second echo pulse; testing the peak amplitude of the second echo pulse; while determining the presence of the second echo pulse at the predetermined minimum amplitude following the holdoff pulse of the preset time; enabling the device should the first pulse be valid and the second echo pulse have a duration greater than the preset time; and further enabling the device for generating the control pulse should the peak amplitude of the second pulse be validated.

14. An apparatus for measuring ultrasonically the thickness of successive sections of an elongated member in which an on signal and an off signal are developed in response to the receipT of first and second associated echo pulses of predetermined test characteristics from one surface and an opposite surface, respectively, of the member and in which relative motion is caused to occur betwen the successive sections of the elongated member and a source of ultrasonic pulses, which includes: means for establishing a test period during which echo pulses are tested; means for transmitting an ultrasonic pulse into the member and then receiving a first echo pulse reflected from the one surface of the member and then receiving an associated second echo pulse from an opposite surface of the member; means for excluding extraneous low level signals; means for testing the first echo pulse and then an associated second echo pulse for a qualifying predetermined minimum amplitude; means actuated in response to receiving a qualifying first echo pulse for initiating the development of the on signal and the examination of a validating duration characteristic of the first echo pulse; means responsive to the validation of the first echo pulse and the receipt of a qualifying associated second echo pulse for initiating the examination of validating test characteristics of a qualifying associated second echo pulse; means for causing the development of the off signal with the time duration between the on signal and the off signal being related to the elapsed time between the receipt of the first echo pulse from the one surface and the receipt of the associated second echo pulse from the opposite surface; means for measuring the elapsed time between the on signal and the off signal; means for generating a pulse to control the disposition of the measured elapsed time; means responsive to the first echo pulse being validated for enabling partially the means for generating a control pulse; means responsive to validating the associated second echo pulse for completing the enabling of the pulse generation means to cause the generating means to generate the control pulse; and means responsive to the generation of a control pulse for converting the measured elapsed time into a signal the amplitude of which is proportional to the thickness of the elongated member.

23. An apparatus for discriminating among pulses received from a sensor to indicate the receipt of a sequence of valid associated pulses, which includes: means for permitting after a predetermined delay the testing of pulses within a predetermined time interval; means for enabling the testing of one parameter of a first pulse upon the receipt thereof; means developing an on signal upon receipt of a first pulse having a valid one test parameter; means for further testing another parameter of the first pulse; the validation of the one parameter and then the other parameter of the first pulse being indicative of a valid first pulse; means for developing an off signal in response to the validation of the first pulse and during the receipt of an associated second pulse; means for measuring a relationship between the on signal and the off signal; means for enabling the testing of one parameter of a second pulse; and means for further testing the second pulse for the validity thereof with respect to another parameter; the validation of the first pulse and the validity of the second pulse with respect to the one parameter enabling a control pulse generator; the validation of the second pulse with respect to the other parameter further enabling the generator; the enabled generator being operated at the end of the predetermined time interval for causing the measured relationship to be further processed as an indication of the receipt of a sequence of valid associated first and second pulses.

24. A logic circuit for validating a sequence of associated first and second pulses, which includes: comparator means for testing the amplitude characteristics of the pulses; means for testing the duration of a first pulse having a required amplitude characteristic; means for testing the duration of an associated second pulse, the testing of the duration of the second pulse initiating the comparator to test the peak amplitude of the second pulse; means for developing an on signal upon receipt of a first pulse having the required amplitude and an off signal upon validation of the first pulse and during the receipt of a second pulse, a predetermined relationship of the on signal and the off signal being related to a parameter to be measured; means for measuring the relationship between the on signal and the off signal; means for causing the measurement to be further processed; first logic means responsive to receipt of a first pulse having the required amplitude for initiating the developing means and the first pulse duration testing means; second logic means responsive to a valid first pulse, a valid first pulse having the required amplitude characteristic only within a preset duration, and a valid associated second pulse, a valid second pulse having a required amplitude characteristic occurring at least within a preset duration for enabling the causing means; third logic means responsive to the validation of a first pulse and to the receipt of a second pulse of required minimum amplitude for enabling the second logic means, for developing the off signal and for operating the second pulse peak amplitude verification means; the validation of the second pulse with respect to peak amplitude further enabling the causing means; and means beginning a test cycle for operating the causing means when the causing means is enabled at the end of the test cycle and for conditioning the circuit for another cycle of operation.

25. An apparatus for measuring ultraSonically the thickness of a cable jacket, which includes: means for cyclically transmitting ultrasonic pulses into engagement with the outwardly and the inwardly facing surfaces of the cable jacket; means for receiving a first echo pulse reflected from the outwardly facing surface and an associated second echo pulse from the inwardly facing surface; first comparator means for testing an amplitude characteristic of the first echo pulse and responsive to a valid first echo pulse for testing the an amplitude characteristic of the second echo pulse, a valid echo pulse having a predetermined minimum amplitude at threshold, and subsequently only occurring within a preset duration, a valid second echo pulse having a minimum amplitude at threshold, the minimum amplitude also occurring within a duration in excess of a preset time, and a peak amplitude greater than a predetermined value; logic means when operated for developing an on signal and responsive to being disabled for developing an off signal with the time duration between the on signal and the off signal being related to the elapsed time between the transmission of a pulse into the outwardly facing surface of the jacket and and receipt of a corresponding echo pulse at the outwardly facing surface after reflection from the inwardly facing surface; means for measuring the time between the on signal and the off signal; a monostable device which when operated causes the time between the on signal and the off signal to be recorded; means delayed a predetermined time for enabling the comparator means and for enabling the monostable device; means for validating the duration of the first echo pulse; means interposed between the first comparator means and the first echo pulse validating means and responsive to the receipt of a first echo pulse having the required minimum amplitude for operating the first echo pulse echo validating means and for causing the developing means to develop the on signal; means for validating the duration of the second echo pulse; second comparator means for testing the peak amplitude of the second echo pulse; switching means rendered effective by the validation of the first echo pulse and enabled by the receipt of the second echo pulse for operating the second pulse validation means; the means for validating the duration of the second echo pulse causing the second comparator means to be operated; switching means rendered effective by the validation of the first echo pulse and the second echo pulse having at least the predetermined duration for further enabling the monostable device; the validation of the peak amplitude of the second pulse still further enabling the monostable device to operate the monostable device and cause the count to be stored; and sequence control means for causing the transmitting means to operate and for conditioning the apparatus for each successive cycle of operation.

26. A logic circuit for discriminating among pulses to determine and record a measurement corresponding to a sequence of valid first and second pulses, which includes: means for delaying the testing of pulses; first comparator means for testing received pulses for minimum amplitude; means connected to the first comparator means for testing a first pulse of a sequence for duration; second comparator means for testing the second pulse of each sequence for a peak amplitude during a duration test thereof; means connected to the first pulse duration testing means for testing a second pulse of a sequence for duration and for enabling the second comparator means; first logic means for developing an on signal upon receipt of the first echo pulse of a sequence and then an off signal upon receipt of a second echo pulse with the time interval therebetween being indicative of a quantity to be measured; means for measuring the time interval; pulse generator means responsive to inputs indicative of a sequence of valId first and second pulses for causing the measured interval to be further processed; second logic means responsive to the receipt of a first echo having the minimum amplitude for causing the first logic means to develop the on signal and to operate the first pulse duration test means; third logic means connected to the first pulse duration test means and the second pulse duration test means and the second logic means for controlling the operation of the second pulse duration test means, and being responsive to a valid first pulse and the receipt of a second pulse spaced from the first for initiating the duration test of the second pulse while causing the first logic means to develop the off signal, the third logic means responsive to an invalid first pulse for inhibiting the operation of the second pulse duration test; fourth logic means connected to the third logic means, the second comparator means and the means for testing the duration of the second echo pulse and responsive to the receipt of a sequence of valid first and second echo pulses for enabling the pulse generation means; and monostable means rendered effective after the delay for enabling the testing of the first and second pulses and responsive at the end of the delay for causing the pulse generator if enabled by receipt of a sequence of valid first and second pulses to generate a control pulse.

27. An electrical logic circuit for developing an on signal and an off signal with the measured time duration between the on signal and the off signal being related to the elapsed time between the receipt of a valid first echo pulse, defined as having at least a predetermined threshold value which does not occur beyond a predetermined time, reflected from a first surface, and the receipt of a valid second echo pulse, defined as having at least a predetermined threshold value which also occurs within a predetermined time and a predetermined peak amplitude being reflected from a second surface spaced from the first surface and for causing the measured time duration for valid first and second pulses to be retained, which includes: comparator means for comparing the threshold values of amplitude of the first and second pulse and the peak amplitude of the second echo pulse; a first echo pulse time duration validation circuit; a second echo pulse time duration validation circuit; means connected to the comparator means for enabling the comparator means to generate an output signal in response to a first echo pulse having the required threshold amplitude; first logic means when operated by the simultaneous impression of like signals on the inputs thereof indicative of a first pulse echo of requisite threshold for developing an on signal and responsive to the receipt of a signal indicative of the receipt of the beginning of the second echo pulse for developing an off signal with the time duration between the on and off signals being related to the time elapsed between the receipt of the first echo pulse and the second echo pulse; monostable means connected to the first and second pulse echo time duration validation circuits for causing the time interval to be retained; bistable means normally in a first state connected to the first pulse validation circuit and the second pulse validation circuit when operated to a second state for operating the second pulse validation circuit; second logic means interposed between the first pulse validation circuit and the bistable means and responsive to a valid first echo pulse for setting the bistable element to the second state; the second pulse validation circuit being connected to the comparator means for causing the comparator means to test the peak amplitude of the second echo pulse during the predetermined duration; third logic means connected to the first validation circuit and the monostable element and operated by the validation of the first echo pulse and the validation of the duration of the second pulse for enabling the monostable means; and bistable means normally in a first unoperated state and when operated by the validation of peak amplitude of the second echo pulse being switched to a second state thereby causing the monostable means to generate a pulse to cause the recording of the time interval.

28. An electrical logic circuit for developing an on signal and an off signal with the measured time duration between the on signal and the off signal being related to the elapsed time between the receipt of a valid first echo pulse defined as having at least a predetermined threshold value which does not occur beyond a predetermined time reflected from a first surface and the receipt of a valid second echo pulse defined as having at least a predetermined peak amplitude and a preset threshold value which also occurs within a preset time duration from a second surface spaced from the first surface and for causing the measured time duration for valid first and second pulses to be processed, which includes: a first comparator for amplitude detection of pulses of either polarity having at least a predetermined level; a second comparator for determining if the peak amplitude of a pulse is greater than a predetermined value; the comparators having an input signal applied thereto in response at least to the receipt of the echo pulses; means operated after a predetermined delay for enabling the comparators; a first bistable element normally in a first state; first echo pulse verification means connected to the first bistable element for determining the validity of the first pulse with respect to duration and that the end of the first echo pulse is spaced from the beginning of the second echo pulse; first logic means responsive to like signals being impressed on the inputs thereof for developing the on signal and responsive to the disabling thereof for developing the off signal; the first logic means for developing on and off signals being connected to the first bistable element; means for enabling the first logic means after the delay; second logic means interposed between the first comparator and the first bistable element and responsive to an echo pulse having the required threshold amplitude impressed upon the input of the first comparator for causing the first bistable element to assume a second state; the setting of the first bistable element to a second state operating the enabled first logic means and causing the first echo pulse verification means to be operated; third logic means connected to the output of the second logic means associated with the first comparator and connected to the first echo pulse verification means; second echo pulse verification means for validating the associated second echo pulse with respect to duration; means for causing the time interval to be processed; means operated upon the inputs thereof being impreseed simultaneously with like signals caused by a valid first pulse and a second pulse of at least the predetermined duration and connected to the causing means for enabling the causing means; a second bistable element connected to the developing means interposed between the third logic means and the second echo pulse verification means and when in a second state for causing the developing means to develop the off signal and for operating the second echo pulse verification means; the second echo pulse verification means being effective during the predetermined time for enabling the second comparator to test the peak amplitude of the second echo pulse; a third bistable element interposed between the second comparator and the causing means and responsive to being set to a second state for further enabling the initially enabled causing means; and the third bistable element being set to a second state by the second comparator receiving a second echo pulse having a peak amplitude greater than the preset value; the causing means being operated by the comparAtor enabling means after a predetermined time greater than the combined durations of the first and second echo pulses plus the separation therebetween.

29. A circuit for measuring ultrasonically the thickness of a jacket of a cable being advanced along a predetermined path with a transducer being spaced from the cable and at least partially immersed in a coupling medium, which includes: means initiating a cycle of operation for causing the transducer to emit ultrasonic waves into engagement with the outwardly facing surface of the cable jacket whereupon successive first echo pulses are reflected, portions of the ultrasonic waves being propagated through the jacket and upon engagement with the interface of the inwardly facing surface causing successive associated second echo pulses to be reflected; a first comparator for determining if an amplitude of successive ones of the associated pairs of first and second echo pulses is at least a predetermined minimum level; ; a second comparator for determining if the peak amplitude of the successive ones of the second echo pulses is at least a predetermined value; a first monostable multivibrator when operated for generating a pulse having a predetermined time interval during which a valid sequence of first and second echo pulses may occur, the first multivibrator being connected to the first comparator; a second monostable multivibrator interposed between the cycle initiation means and the first multivibrator for delaying the generation of the pulse having the predetermined time interval; a third monostable multivibrator responsive to the validation of a sequence of associated first and second echo pulses for generating a control pulse; a valid first pulse having the predetermined minimum amplitude which occurs only within a duration less than one preset time; a valid second pulse having at least a predetermined peak amplitude and a duration in which the minimum amplitude also occurs being in excess of another preset time; the generation of the pulse having the predetermined time interval enabling the first comparator; a first logic element connected to the first comparator and operated by a signal impressed thereon by the first comparator in response to receiving a first echo pulse of at least the required minimum amplitude level; a first echo pulse verification circuit connected to the output of the first logic element; a second logic element connected to the first logic element for developing an on signal and an off signal with the time duration between the on signal and the off signal being related to the elapsed time between the receipt of a first echo from the outwardly facing surface of the cable jacket and the receipt of an associated second echo pulse from an opposite surface of the material; means for measuring the time between the on signal and the off signal; the second logic element being operated when the first logic element is operated for developing an on signal to control the measuring means to begin counting; a third logic element having inputs connected to the first logic element and to the first echo pulse verification circuit; the first echo pulse verification circuit generating a holdoff pulse of the one preset time and for then enabling the third logic element; the third logic element caused to be operated when the first logic element is being operated subsequent to the holdoff pulse and for a predetermined time thereafter; a fourth logic element having inputs connected to the first logic element and to the first echo pulse verification circuit; a second echo pulse verification circuit connected to the fourth logic element; a first bistable device switched in response to the third logic element being operated for discontinuing the operation of the circuit because of the extended duration of the first echo pulse, and switched responsive to the third logic element being in an unoperated state for conditioning tHe fourth logic element; a fifth logic element interposed between the bistable means and the third monostable multivibrator and connected to the second echo pulse verification circuit; the bistable device when switched responsive to the third logic element being unoperated conditioning the fifth logic element; the fourth logic element responsive to the first logic element being operated for impressing a signal on the second verification circuit and on the developing means to cause the developing means to develop an off signal to discontinue the count, the count corresponding to the time between the pulses and related to the jacket thickness; the impression of the signal on the input of the second verification circuit causing the generation of a holdoff pulse of the other preset time; the second verification circuit being connected to the second comparator for impressing thereon a signal to enable the second comparator; a second bistable device normally in a first state interposed between and connected to the second comparator and to the third monostable multivibrator; the enabled second comparator operated by a second echo pulse of the required peak amplitude to cause the second bistable element to assume a second state; the switching of the second bistable element to the second state causing a signal to be impressed on the third multivibrator to enable the third multivibrator a sixth logic element connected to the second verification circuit and to the first logic element; the sixth logic element operated by the simultaneous impression of like signals from the second verification circuit and from the output of the first logic element to establish that the second pulse reaches the minimum amplitude during a time interval in excess of the other preset time; the operation of the sixth logic element causing the enabled fifth logic element to be operated; the operation of the fifth logic element further enabling the third multivibrator; the trailing edge of the pulse having the predetermined time interval operating the enabled third monostable multivibrator to cause the third monostable multivibrator to generate the control pulse which causes the count to be processed to a measurement of thickness; and a pulse repetition sequencer for cyclically operating the initiating means and for resetting the elements of the circuit to condition the circuit for further cycles of operation.

30. Apparatus for emitting ultrasonic waves toward and into engagement with an outwardly facing surface of successive sections of a jacket of a cable being advanced along a predetermined path of travel whereupon a first echo pulse is reflected therefrom and then through the cable jacket and into engagement with the next successive acoustical interface at the inwardly facing surface of the jacket whereat a second echo pulse is reflected, which includes: a transducer aligned with the path of travel of the cable and having at least the portion thereof facing the cable being immersed in a coupling medium; a plurality of serially arranged silicon-controlled-rectifiers which when operated cause a short duration high voltage to be applied to the transducer to excite the transducer and cause the transducer to emit the ultrasonic waves; means responsive to an input for operating the silicon-controlled-rectifiers; means for applying an input to the operating means; means for receiving and amplifying the returning echo pulses; and a diode interposed between the transducer and the silicon-controlled-rectifiers for preventing the amplification of spurious noise signals as generated by the silicon-controlled-rectifiers.