means of through bolts there are two through bolts above the cross slot and two therebelow see fig3 arm 48 is made of several sections bar 50 extends through the cross slot 42 and is carried on pivot pin 52 which extends through straps 44 and 46 to pivotally support arm 48 as seen in fig6 bar 50 is preferably made of metal such as brass so that electric connection can be made thereto as illustrated in fig2 strap 54 is also metallic strap 54 is secured in a slot in the end of bar 50 cap 56 is secured in a similar way on the outer end of strap 54 see fig5 cap 56 has a longitudinal slot 58 therein in which is positioned ball bearing roller 60 pivot bolt 62 passes through roller 60 yoke 64 see fig4 embraces the cap 56 and carries pivot bolt 62 this construction is such that the yoke 64 can move forward and back in the longitudinal direction of arm 48 electrical connector strap 66 is secured to both cap 56 and yoke 64 to provide electrical continuity therebetween the lower end of yoke 64 carries rod 68 which has a connector 70 at its lower.