of the present invention as shown in fig3 the liquid ground contact 44 includes a quantity of electrically conductive liquid 48 held in an upwardly opening recess 50 broadly 34 containment means 34 formed by boring in the upper end of the end cap 46 in the preferred embodiment the liquid 48 is mercury selected in part for its high electrical conductivity in addition mercury is desirable because it does not attack the metal components of the apparatus 42 with which it comes into contact further mercury provides very little resistance to movement so that the moving components of the apparatus 42 in contact with the mercury are not subject to wear an o ring 52 located in an annular groove 54 in the end cap 46 seals with the interior wall 56 of the housing 22 to prevent mercury from leaking out the bottom of the housing the socket headed bolt 28 broadly the 34 grounding contact 34 which secures the ground washer 26 on the shaft 14 projects downwardly from the lower end of the shaft into the recess 50 where it is partially immersed in the mercury 48 although the shaft 14 and bolt 28 rotate relative to the end cap 46 and mercury 48 held.