in fig1 a short - time arc - welding system according to the invention is generally referred to by 10 . the welding system 10 serves to weld elements such as metal studs 12 on to components such as metal sheets 14 . a typical practical example is the welding of fastening studs 12 on to the body panels 14 of motor vehicles . the elements concerned may be metal studs with or without a thread , nuts , eyes , etc . the component 14 may be a body panel , with sheet thicknesses as thin as 0 . 5 mm . the welding system 10 corresponds in its fundamental structure to the short - time arc - welding system which is disclosed in the brochure “ neue tucker - technologie . bolzenschweiβen mit system !” [“ new tucker technology . the stud welding system !”] mentioned at the outset . the information disclosed in this brochure is implicit by reference . the welding system 10 has a control and energy unit 20 to which a plurality of , typically five , feed units 22 can be connected . at least one welding head 24 is connected respectively to the feed units 22 , of which feed units 22 one is represented schematically in fig1 . the control and energy unit 20 has a power supply device to form an arc between the component 14 and the raised element 12 . further the control and energy unit has a main control device . the main control device serves , via suitable interfaces , inter alia to input and display process parameters and to communicate with other processes , for example a production line . the control and energy unit 20 has an interface 26 for the purpose of communicating with other processes . the feed units 22 serve to isolate and to reliably feed elements such as metal studs 12 to the welding head 24 , said feed units 22 each having an isolating device , a pneumatic portion for conveying isolated elements to the welding head 24 , and a feed control device . the welding head 24 receives from the feed unit 22 one element 12 to be welded at a time . for this purpose the welding head 24 has a holder known per se ( not shown in detail ). further a linear motor 28 is provided in order to move the holder — and with the holder the element 12 — in a direction roughly perpendicular to the component 14 on a linear path , as is shown schematically at 29 . consequently by means of the electric linear motor 28 the height h of the element 12 can be adjusted in relation to the component 14 ( lifting height ). further the welding head 24 has a lifting height detection means 30 which detects the respective actual lifting height h . in fig1 the connections between the control and energy unit 20 and the feed unit 22 and between the feed unit 22 and the welding head 24 are shown respectively as a single line . however , it is understood that these connections each contain communication lines for the exchange of information and for transmitting control commands between the control devices of the control and energy unit 20 , of the respective feed units 22 and of the respective welding heads 24 . further the connections contain power supply lines for supplying the welding head 24 with power from the power supply device of the control and energy unit 20 . the connection further contains a compressed air line between the feed unit 22 and the welding head 24 , which compressed air line is for feeding elements 12 that are to be welded on . when providing a distributing guide ( not illustrated ), two or optionally even more welding heads 24 may be connected to one feed unit 22 . the welding heads 24 may be permanently mountable welding heads which , for example , are attached permanently to a robot arm , or else may be freely manipulatable welding guns . the feed units 22 and the welding heads 24 can respectively be adapted to fit different kinds of elements 12 , both in respect of the material characteristics ( for example , steel or aluminum studs ) and in respect of the form ( christmas tree studs , earthing studs , threaded studs , etc .). the welding system 10 as shown represents only one preferred embodiment for use in the industrial field . it is understood that other embodiments could be designed without a special feed unit , the elements 12 to be welded on then being placed in a holder by hand . the operation of the welding system 10 is explained generally hereinafter by reference to fig2 . fig2 shows , applied over time t , the lifting height h of the element 12 in relation to the component 14 , the welding current i , which current flows from the control and energy unit 20 via the feed unit 22 and the welding head 24 through the stud 12 and the component 14 , and the energy quantity e applied to the weld joint . the representation of these variables is of purely qualitative and schematic nature in order to simplify explanation thereof . when carrying out a weld , initially the element 12 is placed on the component 14 so that an electrical contact is formed . subsequently at a point in time t 1 a so - called pre - current i v is switched on . shortly thereafter , at a point in time t 2 , the element 12 is raised in relation to the component 14 , until a point in time t 3 at which the welding lift height is reached . during this time the pre - current remains switched on . an arc is drawn as the element 12 is raised . the pre - current arc has sufficient energy to burn off contaminants present in the region of the welding point or to evaporate possible layers of wax . after reaching the welding lift height , at time t 4 , the welding current i s is brought into circuit . as a result of the high welding current i s of approximately , 20 a to 1 , 500 a for example , the element 12 and the component 14 are melted in the region of the drawn arc . the energy e flowing into the welding operation increases . the lowering operation is initiated at a point in time t 5 . the element 12 is lowered at a predetermined speed until at a time t 6 it impinges on the component 14 . at this point in time , the arc is short - circuited and the welding current i s is switched off . as shown qualitatively in fig2 , the lowering operation can take place until slightly below the zero line , in order to guarantee a saturated immersion of the element 12 in the melted surface of the component 14 . the melts of element 12 and component 14 unite and cool down owing to the interruption in the energy input . the total energy quantity applied is designated by e end in fig2 . the element 12 is thus welded firmly on to the component 14 and may serve as an anchor for fixing parts to the component 14 . in fig2 the total welding time t s is shown , i . e . the time from t 4 to t 6 . the pre - current time t v is represented as the time from t 1 to t 4 . finally a lowering time t a is visible , being the time from t 5 to t 6 . the welding operation described by reference to fig2 corresponds to the prior art . to raise and lower the element 12 , either a linear motor or a spring - mass system ( with an electromagnet and counter spring ) can be used . in particular when using the electric linear motor 28 , the lifting height h can be regulated by setting a certain desired profile for the lifting action and causing the measured lifting height h to be regulated to the respective desired profile . this makes it possible to obtain an accurate positional definition between the element 12 and the component 14 at each point in time of the welding operation . the current i is made available from a constant - current source from the power supply device of the control and energy unit 20 . the arc voltage between the element 12 and the component 14 consequently is adjusted according to the electrical resistance present between these two elements . the electrical resistance depends decisively on the surface characteristics , contaminants , etc . and can vary from welding operation to welding operation . from the wo 96 / 10468 mentioned at the outset , it is therefore known to regulate the lifting height h during the welding operation such that the arc voltage between the element 12 and the component 14 is constant or follows a predetermined finished profile . by means of this system it is possible to maintain the arc voltage constant during the welding operation and thereby to obtain smaller fluctuations in welding quality from welding operation to welding operation . details of the regulation process are disclosed in wo 96 / 10468 , the disclosure of which herein is fully implicit by reference thereto . in this document it is also described that the lowering operation takes place at a certain lowering speed which is set beforehand . in the prior art , this can mean that the total welding time t s varies from welding operation to welding operation . if , for example , in the representation in fig2 the lifting height h at the time t 5 is greater than that shown , the time t 6 will shift backwards in time because the lowering speed is constant . consequently the total welding time t s will also be lengthened . if by contrast the lifting height is less than that shown in fig2 at the time t 5 owing to the regulation of the lifting height for adjusting the arc voltage , the time t 6 will shift forwards in time , which leads to a shortening of the total welding time t s . it is understood that the energy input e will also alter correspondingly depending on lifting height h at time t 5 . a preferred embodiment of the arc - welding process according to the invention is shown in fig3 . in fig3 , on the one hand , the profile of the lifting height h is presented as reference , as is also shown in fig2 . in order to obtain a constant total welding time t s , with otherwise unchanged boundary conditions , the welding operation is terminated at t 6 , irrespective of the lifting height h last adjusted during the welding operation owing to closed - loop control . if the element 12 for arc voltage regulation is raised , for example , in the course of the welding operation , as is shown schematically at h ′, subject to a constant lowering speed , the initiation of the lowering operation will already take place at a time t 5 ′ which lies prior to the point in time t 5 . if , in contrast , during the regulation a lower lifting height is set , as is shown schematically at h ″, the lowering operation will only be initiated at a point in time t 5 ″, which lies chronologically after the point in time t 5 . the maintenance of a constant lowering speed starting from the initiation of the lowering operation until the point in time t 6 has the following advantages . the lowering speed can , on the one hand , be chosen to be high enough in order to prevent melt dripping from the element 12 on to the component 14 before immersion . on the other hand , the lowering speed can be selected to be sufficiently slow in order to prevent a hard impact on the component 14 which impact could lead to a rebounding and splashing of melt . the value of the set predetermined lowering speed in the case of this embodiment is consequently a compromise between these two boundary conditions . fig3 further shows that the welding process according to the invention can also be used if the lifting height h is not regulated . however , it may be that in different instances different lifting heights are desired during the welding operation , as shown in fig3 by h , on the one hand , and by h ′″, on the other hand . conventionally , a different shutdown time t 5 or t 5 ′″ respectively would have to be programmed for the various lifting heights h and h ′″. in the process according to the invention , the correct point in time t 5 or t 5 ′″ is selected automatically by means of the detected lifting height , thereby rendering a special programming of this point in time unnecessary . fig4 shows an alternative embodiment of the welding process according to the invention . in this embodiment the regulation of the lifting height is always up to a fixed point in time t 5 , irrespective of the lifting height h a or h b present at the time t 5 . in order nevertheless to achieve a set predetermined total welding time , the speed of the lowering operation is selected such that the lowering operation always terminates at a point in time t 6 . in the example shown in fig4 the lifting height on the curve h a is thus relatively high at the point in time t 5 . consequently a relatively high speed must be set in order to terminate the total welding operation at the point in time t 6 . if , on the other hand , the lifting height at the point in time t 5 is relatively low , as is shown on curve h b , the speed to be selected to reach the total welding duration will be relatively low . this alternative embodiment of the process according to the invention is slightly simpler to program . in contrast , the embodiment of fig3 has the advantage of a uniform lowering speed and thus a defined immersion operation . a further embodiment of the process according to the invention is presented in fig5 . in this embodiment , similarly to the embodiment in fig4 , the regulation of lifting height during the welding operation always takes place by the point in time t 5 . in order subsequently to achieve a constant total welding time t s , not only is there action to influence the speed of the lowering operation , but also to influence the acceleration thereof . thus by means of the course of the lifting height action h x it can be detected , for example , that directly after the point in time t 5 the lifting height is reduced initially at a relatively high speed and the speed while approaching the point in time t 6 is continuously reduced in order to achieve a gentle immersion operation . it is understood that this manner of influencing the lowering operation can also be used with the embodiment in fig3 , in that , instead of a continuous lowering speed , a certain path - time profile of the lowering operation is predetermined , which path - time profile , for example , can take on the form of a parabola , as is shown in fig5 . in fig5 a further lifting curve h y is shown wherein the lowering operation is also initiated at a point in time t 5 and is terminated at a point in time t 6 . in this embodiment , to achieve an especially hard impact on the component 14 the lifting height after initiating the lowering operation at the point in time t 5 is still maintained comparatively high , and , while approaching the point in time t 6 , the element 12 is then accelerated towards the component 14 . it is understood that the embodiments in fig3 and 5 can also be mutually combined in order to complete a welding operation with a certain total welding duration and / or a certain total welding energy . furthermore it is understood that the process according to the invention can also be applied to short - time arc - welding processes in which the arc is not “ drawn ”.