referring to fig1 and 2 , a system of the invention requires two separate welding heads 10 and 12 which are placed as close together as possible , and are electrically insulated from one another by an interposed piece of insulation 14 . the requirements for gas - shielding of the arcs in operation will be apparent to those skilled in the art and do not require specific description herein . the heads are supplied with respective consumable electrode wires 16 and 18 from reels 20 and 22 driven by controllable motors that are not shown . the head 10 is designated the master head and is supplied with pulses of welding current superimposed on a background current from a controlled master power supply 24 , so as to produce a stable master arc between its electrode wire 16 and a workpiece 26 . the head 12 is designated the slave head and is supplied with pulses of welding current superimposed on a respective background current from a controlled slave power supply 28 , interconnected at 30 with the master power supply , thereby producing a stable slave arc between its electrode wire 18 and the workpiece 26 . the wave forms of the master and slave pulses are shown respectively in the lower and upper parts of fig2 the pulses being of peak values im ( master ) and is ( slave ), and of duration tm ( master ) and ts ( slave ). the values of the respective background currents are indicated by bm ( master ) and bs ( slave ) and are independent of the pulse peak values . in this embodiment the pulse durations are somewhat shorter than the respective pulse intervals and the two series of pulses are exactly 180 degrees out of place , so that each slave pulse occurs during an interval between two successive master pulses with its leading edge trailing a time delay period td after the leading edge of the immediately preceeding master pulse . other pulse durations , intervals and delays will be employed in other embodiments , to be determined by the operator . for example , although the pulses are illustrated herein as completely rectangular in profile , in practice the leading and trailing edges will be sloped and some overlap can therefore be tolerated between the trailing edge of one pulse ( master or slave ) and the leading edge of the immediately succeeding pulse ( slave or master respectively ) if their duration is larger than as illustrated and / or if the phase difference is not exactly 180 degrees . the master power supply 24 is supplied by the operator via controls indicated by 32 with suitable values for a reference voltage determining the master arc length , the master pulse parameters and the time delay td . the slave power supply 28 may be supplied by the operator via controls indicated by 34 with a reference voltage for the slave arc length and pulse parameters for occasions when the slave arc is to be operated asynchronously from the master arc , at which time the interconnection 30 is not effective . the system comprises a master arc voltage feedback control system 36 , as disclosed in our prior u . s . pat . no . 4 , 620 , 082 , connected between the master head 10 and the master power supplied 24 , sensing the master arc voltage and modulating the frequency of the master arc pulses to establish the frequency , and thus the average current required to maintain the master arc stable . the signal from the system 36 is also used to modulate the frequency of the slave arc pulses so that the two sets of pulses will remain in sychronism with one another with the pre - set time delay td between them . there is no guarantee that this frequency modulation of the slave pulses will maintain the slave arc stable , and accordingly the system is also provided with a slave arc voltage control system 38 connected between the slave head 12 and the slave power supply and modulating the slave pulse duration ts and / or the slave background current bs so as to maintain the average current at a value that will stabilize the arc . the two sets of pulses can therefore remain exactly out of phase with one another so as to minimize &# 34 ; arc blow &# 34 ; magnetic interference between the two arcs , while maintaining them both adequately stable . the electrodes can therefore be placed in very close proximity , as close as only 3 mm apart , more usually about 5 mm apart , so that successive weld beads can be laid successfully within a narrow parallel - walled channel between the workpieces to be joined , without requiring excessive spacing between the workpieces and without the need for mechanical movement of the wires and / or the heads , other than the translational movement required to lay the beads in their desired paths . an examination of the operation of the system by high speed photography ( 3 , 000 frames / sec ) shows successive establishment of the two arcs that is an electric switching equivalent of the mechanical systems used hitherto , without the problems of design and maintenance involved with such systems . this overcomes the magnetic field interactions and produces a very stable metal transfer condition on each electrode . the system of the invention provides the possibility of three arc gas - shielded metal arc welding with the master electrode disposed between the other two which constitute slave electrodes both controlled as described for the single slave electrode . fig3 shows the typical &# 34 ; finger &# 34 ; penetration profile obtained with a single electrode operated to obtain spray transfer . good penetration is obtained at the centre line , but the edges do not exhibit substantial penetration into the base metal . the deep penetrating arc can also lead to &# 34 ; burn - through &# 34 ; when welding thinner materials or if the workpieces are not properly set - up fig4 shows the profile obtained in a bead - on - plate weld with two electrodes of 0 . 9 mm steel with a transverse separation of 5 mm . the wire feed speed was 106 mm / s ( 250 ipm ) for each electrode which corresponds to a total average current requirement of about 200a - 230a . pulse parameters were identical on each electrode ( peak current = 275a ; background current = 40a ; pulse duration = 4 . 0 ms ) and the delay time was 5 . 0 ms . the travel speed was 3 . 8 mm / s or about 9 ipm . it will be observed that the depth of penetration is much more uniform across the bead width and , although the magnitude of penetration is reduced , it exhibits lobes at the ends that are evidence of independent operation of the two welding arcs , so that it is particularly satisfactory for application to narrow gap welding to ensure good side wall fusion and penetration . it is noteworthy that deposition rates equivalent to those achieved with a single wire are obtained at twice the arc travel speed , implying the possibility of doubled productivity in commercial applications . because of the substantially independent but synchronous control of the two arcs , the use of two different consumable electrodes 16 and 18 is permitted . for example , it can be used in applications where a weld deposit of a particular chemistry is required , but no single commercial electrodes are available , or in applications where it is desired to custom - alloy in the weld pool from two different electrodes . the electrodes can be of two ( or three ) different sizes and the wire feed speeds do not need to be identical . each electrode can have different pulse parameters , while the electrode separation is easily variable and the electrode orientation can easily be adjusted . in an experimental system that was employed for test purposes , the two heads 10 and 12 were electrically connected together , a pulse of current being applied to both electrodes simultaneously . since they share the current conductor there is no control over the current distribution between these two electrodes , and although on average , over a period of time , each electrode transmits half of the applied current , the instantaneous current distribution can differ substantially between the two electrodes . these inconsistencies of current distribution can cause non - uniform melting , droplet growth and irregular transfer of the metal . it was found when welding a common conductor and a single power source arrangement that a common arc column clearly emanated from the two electrodes , and this phenomena could be expected from considerations of arc - blow . with the electrodes in such close proximity , there is also the possibility of the molten droplets from each wire joining to bridge the gap . once the gap is bridged , the current distribution may change so that one electrode conducts all of the current , and after droplet detachment the arc would only be present on one electrode . the relatively random nature of current distribution leads to non - uniform melting rates and promotes inconsistencies in droplet size and the mechanism of metal transfer . other advantages of the method and apparatus of the invention are that they can provide the same benefits as manual ( or automatic ) weaving to control bead shape or to accommodate variation in joint fit - up . the more uniform penetration profile that is obtained gives greater uniformity to the heat - affected zone profile , which has positive implications where temper - bead techniques are used in repair welding situations . controlling penetration profile also provides a measure of control over the dilution with the parent material , which has potential benefits in weld - surfacing operations where minimum dilution is desirable . they offer potential advantages in reducing interbead lack - of - fusion in multi - pass welds as a result of being able to modify the &# 34 ; finger &# 34 ; penetration profile to a more uniform shape . a particularily advantageous application of the system is for narrow - gap applications , since it offers greater assurance of eliminating sidewall fusion defects . thus , narrow - gap gmaw has not yet received extensive industrial acceptance because of the sidewall fusion defect incidence that has been hitherto experienced with the single wire gmaw process . submerged - arc welding has more widespread acceptance because it is less prone to fusion defects , but it has the drawback of requiring removal of the fused flux before subsequent passes are deposited . the reliability of the narrow - gap gmaw process provided by this invention should lead to its increased adoption . fig5 is a cross - section through a partially completed narrow gap weld produced using the apparatus of the invention and its excellent side / penetration characteristics will be apparent to those skilled in this art . this weld was produced in a parallel - sided joint with identical parameters to that used in the head - on - plate weld of fig4 . fig6 shows schematically , but in more detail the electrical circuit of the apparatus of the invention ; parts thereof which are similar in the master and slave circuits are given the same reference number with the respective subscript m and s . the apparatus is supplied with three phase a . c . current from a suitable power source which is fed to a full wave rectifier bridge 40 , the d . c . output being smoothed by capacitor 42 . the smoothed d . c . is fed to respective bridge inverter circuits 44m and 44s where it is inverted to a 16 khz square wave and stepped down in voltage to the usual arc - operating value of 40 volts . the square wave frequency is chosen to be above the usual audible range , so that audible hum from the arc , the transformers and similar magnetic components is minimized , the frequency also being much higher than the usual pulse repetition frequency , which is varied principally in dependence upon wire speed and typically is of the order of 30 - 200 hz . the higher frequency permits the use of smaller components in the circuits , particularly in the heavy current transformers that are required . this square wave is then rectified by a respective fast recovery diode full wave rectifier 46m and 46s and supplied through respective inductance 48m and 48s to the respective welding head 10 and 12 . respective arc voltage signals are obtained between the electrodes 10 and 12 and the workpiece 26 and are fed to a respective reference current block 52m and 52s , these blocks also comprising controls , shown as variable resistors , by which the operator can set the reference arc voltages ( 54m nd 54s ), the peak arc pulse amplitudes ( 56m and 56s ), the amplitude of the background currents ( 58m and 58s ) and the duration of the arc pulse peak current ( 60m and 60s ). each reference current block controls a respective pulse width modulation circuit 62m and 62s , that is fed with a respective current signal obtained from a respective resistive shunt 50m and 50s connected in the current path to each welding head , the pulse width modulation circuits in turn controlling a respective power transistor turn on / off circuit 64m and 64s that controls the current supplied by the respective inverter 44m and 44s . thus current regulation is obtained by controlling the periods for which the transistors in the inverter circuit are turned on . the reference current signal from each also consists of a pulse superimposed on a background value , and the frequency of the pulses is controlled by comparing the arc voltage and the reference arc voltage , the resulting error signal driving a voltage controlled oscillator , the output of each oscillator triggering a respective timer that provides a pulse of pre - set duration . an increase in the reference arc voltage results in an increase in the frequency of the pulses to give a higher average current and a longer arc length , and vice versa . an increase in wire feed speed decreases the arc length and results in an increased pulse frequency with corresponding increased output current and increased wire melting , so that arc length variations are minimized . so far , both master and slave power supplies are identical and they can operate independently of one another ( asynchronously ) if required . the master reference current block 52m is selectively connected to the slave reference current block 52s and when so connected each slave arc pulse is triggered upon reception of a corresponding master arc pulse after the delay td , as described above . the frequency of the master and slave pulse trains are therefore the same dependence upon the master arc voltage . the slave arc voltage signal is fed to the current block 52s is used therein to produce an error signal that reduces the slave pulse peak duration and / or its background amplitude if it is greater than the reference slave arc voltage , thereby reducing the average slave arc current and slave arc length ; similarly if the slav arc voltage is lower than the reference voltage the error signal causes the increase in the slave pulse peak time duration and / or the background current amplitude sufficient to stabilize the average arc current and arc length within desired limits . fig7 shows in more detail the circuits of the master and slave reference current blocks 52m and 52s . in some respects , as will become more evident , the two circuits are identical , while they differ in others . the master arc voltage signal is fed to one input of an error signal differential amplifier 66m , the other input of which is supplied with the master reference arc control voltage from potentiometer 54m . the amplifier will produce an error voltage that varies about a mean value in dependence upon whether the arc voltage is larger or smaller than the predetermined value , and this error signal is supplied to a voltage controlled oscillator 68m whose output is a square wave of frequency dependent upon the input voltage . the oscillator output is fed to pulse generator 70m which generates output pulses in response to the input , the width or duration of these output pulses being pre - set by variable resistor 60m . the two generators 70m and 70s are identical and both provide for pulse width modulation by an external control signal , but this facility is not needed in the master generator . the output of generator 70m is fed via pulse current control potentiometer 56m to one output of an adder circuit 72m , from which it is fed to master pulse width modulation circuit 62m that is supplied with an arc current signal from shunt 50m to enable the circuit to control the current supplied by inverter 44m and rectifier 46m and thereby stabilise the arc . the output pulses are also fed to the base of a transistor 74m connected in series with resistor 76m , the latter being in series with background current control potentiometer 58m via diode 78m . the transistor 74m is switched on for the duration of the pulses from the generator 70m , when it short circuits the potentiometer 58m , so that no signal is supplied therefrom to the adder 72m . in the absence of a pulse , resistors 76m and 58m form a potential divider supplying a background current level control signal to the adder 72m . the pulse and background currents signals therefore separated from one another and can be adjusted independently without a change in one causing a change in the other . the identical items of the slave current reference block have the same reference with the subscript s and operate identically , so that their operation has already been described . the output of the master oscillator 68m is fed to a pulse delay circuit 80m whose output is supplied via switch 82m to the input of pulse generator 70s ; the pulse delay provided by the circuit 80m is controllable by variable resistor 84m and is used to adjust the relative phase of the master and slave pulses , as described above . the circuit 80m also includes an optocoupler to electrically isolate the two circuits . with switch 82m in this position , the output of oscillator 68s is ineffective and the two circuits operate in synchronism , while in its alternative position the two are separated and oscillator 68s and pulse generator 70s are connected , so that the circuits operate independently and asynchronously . in synchronous mode , switch 82s is closed and the error signal that in asynchronous mode would control the oscillator 68s instead controls an error signal conditioner circuit 86s . in this embodiment , the circuit 86s produces two output signals one of which is fed to pulse generator 70s as the pulse width modulation signal described above , while the other is supplied to the base of transistor 88s to control the background current level . thus , so long as the transistor 74s is not conducting the transistor is operative in series with resistor 76s as a potential divider able to control the background current signal fed to the adder circuit 72s . both slave current pulse width modulation and slave background current level control have the same effect of changing the average arc current , and are controllable independently of one another . it is therefore possible to use either of them for control of the slave arc average current , but more flexible and satisfactory control is obtained if both modes are employed simultaneously .