having described fig1 thru 4 which relate to the prior art , attention is now directed to fig5 thru 7 , wherein like reference numerals designate identical or corresponding parts throughout the several views , and more particularly to fig5 thereof , wherein one embodiment of the present invention is illustrated . as will be apparent from a comparison of fig1 and 5 , the apparatus illustrated in fig5 contains many elements which are identical to the apparatus of fig1 . more particularly , fig5 includes a working electrode 1 separated by a working gap from a workpiece 2 . a plurality of transistors 3a , 3b . . . 3n are coupled together in parallel , as in fig1 . again , the number of transistors used depends upon the magnitude of the current required in the working gap . resistors 4a , 4b . . . 4n and 5a , 5b . . . 5n are coupled to the collector and base electrodes , respectively , of transistors 3a , 3b . . . 3n , as in the fig1 apparatus . a timer circuit , or timer - counting circuit 6 , is coupled through an amplifier 7 to each of resistors 5a , 5b . . . 5n . a dc voltage source e o is coupled between working electrode 1 and the emitter circuits of transistors 3a , 3b . . . 3n . all of these elements are also illustrated in fig1 . however , the apparatus of the present invention , as illustrated in fig5 includes the following additional elements . a shunt resistance 17 is coupled between one terminal of dc voltage source e c and the emitter electrodes of transistors 3a , 3b . . . 3n . a discharge initiation detector 18 is coupled across shunt resistor 17 . a no - load voltage impressing timer 19 is coupled between discharge initiation detector 18 and an electrical discharge duration controller 20 , which , along with a quiescent length tester 21 , is included in timer 6 . the discharge initiation detector 18 , no - load voltage impressing timer 19 , and timer 6 comprise a control device , designated 23 . in operation , a working voltage is impressed across the working gap between electrode 1 and workpiece 2 by switching transistors 3a , 3b . . . 3n to their conductive state by means of an appropriate signal from timer 6 acting through amplifier 7 . after a brief delay period from the time at which voltage is first impressed across the working gap , an electrical discharge occurs in the working gap and current flows through shunt resistor 17 . the electrical discharge initiation detecting circuit senses the voltage drop across shunt resistor 17 due to the current flowing through it , and thereby detects the initiation of an electrical discharge in the working gap . the no - load voltage impressing time , which is the time between the instant at which a voltage is impressed across the working gap and the instant at which a discharge is initiated in the working gap is then measured by the no - load voltage impressing timer 19 . the duration of the electrical discharge is controlled by the electrical discharge duration control circuit 20 . this circuit is time controlled , and switches transistors 3a , 3b . . . 3n to their non - conductive state after a specified period of time . the quiescent length tester 21 determines the interval during which transistors 3a , 3b . . . 3n are maintained in their non - conductive or open circuit state . it will be noted that quiescent length tester 21 is coupled through a terminal 22 to amplifier 7 . the duration of the electrical discharge produced by this apparatus can be controlled by the no - load voltage impressing time using a control apparatus to be described hereinafter . in accordance with the operation of this control apparatus , when an electrical discharge occurs at the same instant that a working voltage is impressed across the working gap , that is , when the voltage across the working gap does not reach the no - load voltage level during a working pulse , the electrical discharge duration is minimized , thereby reducing the average processing current . the control apparatus can also limit and control the maximum duration of the electrical discharge in the working gap by increasing the no - load voltage impressing time , as is described below . the control device 23 is illustrated in greater detail in fig6 . as described above , a terminal 22 of control device 23 is coupled through amplifier 7 to transistors 3a , 3b . . . 3n . these transistors are in their non - conductive state when a signal is applied to terminal 22 and they are in their conductive state when no signal is applied to the terminal 22 . control device 23 includes a transistor 24 which is coupled to the output of the electrical discharge initiation detector 18 . transistor 24 is controlled by the electrical discharge initiation detector 18 such that it is in its non - conductive state only during an electrical discharge in the working gap . a zener diode 25 is coupled through a base resistor 26 to terminal 22 and is also connected to the base of a transistor 27 . zener diode 25 is in its conductive state when a signal is applied to terminal 22 and is in its non - conductive state when no signal is applied to terminal 22 . accordingly , when a signal is applied to terminal 22 , transistor 27 is switched to its conductive state by the current flowing through base resistor 26 and zener diode 25 . the collector electrode of transistor 27 is coupled through a resistor 31 to the collector electrode of transistor 34 , and is also coupled through a capacitor 32 to the positive terminal of a dc source 29 or e 1 . the emitter electrode is connected to the negative terminal of dc source 29 . a second dc source 30 or e 2 is connected at its positive terminal to the negative terminal of dc source 29 and is connected at its negative terminal to the emitter electrode of transistor 24 . a third dc source 33 or e 3 is coupled at its positive terminal through a resistor 34 to the collector electrode of transistor 24 and is coupled at its negative terminal to the positive terminal of dc source 29 , the emitter electrode of a transistor 36 and to the emitter electrodes of two other resistors housed in a monostable multivibrator 37 . the collector electrode of transistor 27 is coupled through a terminal point 28 to a resistor 35 which is coupled to the base of a transistor 36 . transistor 36 is coupled at both its collector and emitter electrodes to monostable multivibrator 37 . in the operation of this circuit , transistor 24 is in its conductive state and transistor 27 is in its non - conductive state when no electrical discharge is produced in the working gap . in this condition , the electric potential of the terminal point 28 drops to a constant level which is determined by the values of resistor 31 and condenser 32 , by the polarity of dc source 29 (- e 1 ) and by the sum of the electric potentials of dc power sources 29 and 30 [-( e 1 + e 2 )]. when an electrical discharge exists in the working gap , transistors 24 and 27 are both switched to their non - conductive states . accordingly , capacitor 32 charges , and the potential at terminal 28 increases to a level which is determined by the values of resistor 31 and capacitor 32 and by the electric potential of dc source 33 ( e 3 ). after a certain time delay , during which the electric potential of the terminal 28 becomes positive , transistor 36 is switched to its conductive state through the base resistor 35 , whereby monostable multivibrator 37 is actuated . this causes transistors 3a , 3b . . . 3n to be switched to their non - conductive state so that the voltage applied across the working gap is reduced to zero . after a predetermined time delay , the monostable multivibrator 37 returns to its initial condition , and voltage is again impressed across the working gap . if the no - load voltage initially impressed across the working gap is maintained for a sufficiently long interval , capacitor 32 charges to its maximum negative voltage , namely , -( e 1 + e 2 ) volts , and remains constant at this voltage level . the manner in which the control device illustrated in fig6 regulates the duration of the discharge in the working gap is illustrated more clearly in fig7 a , 7b and 7c . it will be recalled that fig7 a illustrates the voltage waveform existing in the working gap , while fig7 b illustrates the current waveform existing in the working gap . fig7 c illustrates the potential of the terminal 28 in fig6 which is equivalent to the voltage on capacitor 32 . referring to fig7 c , the numeral 38 designates the electric potential of the terminal 28 during the no - load voltage impressing time . the numeral 39 designates the electric potential of the terminal 38 during an electrical discharge in the working gap . the numeral 40 designates the electric potential of the terminal 28 during the quiescent period . as will be clear from reference to fig7 the voltage or potential or terminal 28 is set by the circuit illustrated in fig6 at - e 1 during the quiescent period 9 . during the no - load voltage impressing time 10 , the potential at terminal 28 decreases as illustrated at 38 due to negative charge accumulating on capacitor 32 . if the no - load voltage impressing time is short , only a small additional negative charge will accumulate on capacitor 32 . however , if the no - load voltage impressing time is long , capacitor 32 will charge to its maximum negative voltage value of -( e 1 + e 2 ), and will remain at this value until a discharge occurs in the working gap . when a discharge occurs in the working gap , capacitor 32 will discharge as illustrated at 39 until the potential at terminal 28 reaches zero volts . at this time , transistors 3a , 3b . . . 3n are switched to their non - conductive states , and the working pulse is terminated . the physical significance of the mode of operation just described is as follows . if the no - load voltage impressing time is long enough to permit capacitor 32 to charge to its maximum negative voltage of -( e 1 + e 2 ), the duration 11 of the discharge in the working gap will reach its maximum value . this maximum value is determined by the time required for capacitor 32 to charge from its maximum negative voltage value of -( e 1 + e 2 ) to zero volts . if the no - load voltage impressing time 10 is longer than the time required for capacitor 32 to charge to its maximum negative voltage value of -( e 1 + e 2 ), the duration 11 of the discharge in the working gap will still remain at its maximum value , since the same amount of time is required to discharge capacitor 32 to zero volts . however , if the duration of the no - load voltage impressing time 10 is less than the time required to charge capacitor 32 to its maximum negative voltage , the duration 11 of the discharge in the working gap is reduced , since then the duration 11 of the discharge in the working gap only continues for a period of time required to charge capacitor 32 from a voltage between - e 1 and -( e 1 + e 2 ) to zero volts . this period of time is , of course , less than the maximum discharge duration . as a result , it is clear that the control device of fig6 reduces the duration of the discharge in the working gap in response to decreases in the no - load voltage impressing time . reduction of the discharge duration also reduces the average processing current . since the length of the no - load voltage impressing time is a measure of the condition of the working gap as described above ( a measure of the accumulation of waste powder , for example ), the control apparatus 23 effectively controls the average processing current in response to changing conditions in the working gap . the effect of so regulating the average processing current is to improve the condition in the working gap and generally maintain the stability of the workpiece shaping process . it will be noted that it is not necessary to decrease the voltage across the working gap to zero volts during the quiescent period . it is necessary only to reduce the voltage sufficiently to stop the discharge current in the working gap . in addition , the no - load voltage need not be a constant voltage , but may consist of multiple high voltage levels or impulse voltages . it is also possible to use a device which detects the end of the no - load voltage period in place of the device for detecting the initiation of electrical discharge in the working gap . obviously , numerous additional modifications and variations of the present invention are possible in light of the above teachings . it is therefore to be understood that within the scope of the appended claims , the invention may be practiced otherwise than as specifically described herein .