A coil spring 21 acts against the plunger 19 to urge the same to its or contact open position but is normally restrained from doing so by a mechanical latch mechanism generally indicated at 23 which mechanism acts to retain the plunger in either its “in” or its “out” position.56 The mechanical latch which is shown, for purposes of illustration only, is of the type embodying lever like members 25 which are pivotally supported in the housing 9 and engage balls 27 which in turn are supported in openings in a stationary sleeve 26,surrounding the plunger adjacent the bottom of the housing 9.