a further flat surface 23 which can freely slide along the rack moved by another piston operated by a pneumatic control system 24 on this flat surface 23 a motor 25 is provided for causing rotation of a pin 26 having a threaded end said pin is so shaped as to cross the front bar of the locking device 9 as well as to cross the first electrode 7 and to come out from the front side of said portion 7 39 the axis of pin 26 coincides with the axis of a hole 35 provided on the platelike body 30 as seen in fig4 the motor 25 can be operated to cause the sliding of the flat surface 23 such that the pin 26 will be screwed into the hole 35 of the platelike body when the screwing operation begins the pistons 20 of the device 4 prevent the platelike body from moving the pin 26 presents a part of an enlarged section 27 adapted to act against the front bar of the locking device 9 which is moved by another pneumatically controlled piston 28 once the drawing out device has completed the screwing process described above the piston 28 carries back the locking device 9 whose front bar acts against the enlarged section 27 of pin 26 to pull it backward the front bar of the locking device is a portion of a frame 40 which is connected to the piston 28 shifting is completed when the platelike body is joined to the front side 7 39 of the first electrode 7 in order to reduce the electric resistance between the platelike body and the first electrode the adherence acquires a value for example some hundreds of kg the insertion of the platelike body into the.