in such a way that the coupling boards 17 carrying junctions with the insulating board 16 are held in contact with the lappet bars 10 under pressure by the action of the adjacent lappets as is clear from fig8 the circuit boards 15 are disposed on the lappet installation side the front face of the lappet bars 10 and sandwiched between the insulating boards 16 by means of lappet installing bolts and nuts by the nuts 102 fig2 the lappet installing bolts not shown are fastened to run through the hole 132 fig5 in the lappet bracket 13 a hole 162 formed in the insulating board 16 a hole 151 in the circuit board 15 and an installing hole 101 in the lappet bar 10 each circuit board 15 carries a printed circuit pattern on one major surface of an electrically insulating material although the drawings illustrate these circuit boards 15 and insulating boards for six weight use it is obvious that they are equally applicable to four to eight weight uses electrical connections 18 leading to the terminals 14 are disposed above the installing holes 151 in the circuit boards 15 respectively above the installing hole 162 in the insulating board 16 there is formed an elongate slot 161 through which the terminals 14 extending backwardly from the lappet 3 run when the lappets are installed as stated above the terminals are in contact with the connections 18 to establish electrically conductive paths via the elongate slot 161 the circuit board 15 is electrically connected as depicted in fig6 taken on the lines a a and b b of fig2 the circuit board 15 carries electrically leading symmetric regions 19 at the both ends thereof.