of current may reach up to 40 000 amps and there is no need of especially advanced contact equipment water cooled cups as shown in fig2 b with the edges or if desired all four corners of the blank are in contact with the contact cups proved to be sufficient the edges or if desired corners of the blank are the first to be heated and to come to rest at the pressure exterted by cylinders ensuring contact pressure the edges and if desired corners will thus get a gradually increasing face of contact with the cups the secondary switch as shown in fig2 a comprises two contact members 6 39 6 34 which are provided on a control spindle 6 34 39 at one end of the spindle a pressure cylinder 13 is provided which acts on spindle 6 and will urge contact members 6 39 6 34 towards each other and into contact with current supply members which extend to transmission contacts 4 helical springs 14 are inserted between contact members 6 39 6 34 and an insulating centering piece 15 between current supply members causing contact members 6 39 6 34 out of contact with current supply members when the secondary switch is disconnected a conveyor or manipulator 7 as shown in fig3 5a 5b and 6 is provided for insertion and removal respectively of blanks 1 in transmission contacts 3 4 in chamber 5 through an opening 9 9a in the wall surrounding said chamber manipulator 7 comprises gripping members 8 for movement to and from blanks 1 normally on the latter for insertion removal of the blanks in transmission contacts 3 4 in chamber 5 in an embodiment shown.