Abstract: A mercury switch which operates the speed control of a circular hosiery knitting machine is positioned within an elongated chamber of a switch housing. The switch housing, in turn, is pivotally mounted to the frame of a knitting machine at such a position as to be operated by a lever responsive to progression of the pattern chain and/or cam drum, and alternately tilted between a "switch activated" and "switch deactivated" position. The mercury switch is so arranged within the housing that when the housing is lowered to the deactivated position, the mercury is disengaged from the contacts of the switch, and when the housing is raised to the activated position, the mercury is caused to move by gravity down into engagement with the contacts of the switch. The mercury switch is completely sealed within the housing, so that oil, oil vapors, dust and the like cannot work thereinto.