Abstract: A switch is responsive to fluid flow within a conduit. It is highly sensitive to variations in flow and forms an effective seal between the fluid and atmosphere. It includes a plastic pivot ball in a spherical seat. A blade extends from the pivot ball into the conduit where it can be displaced by fluid flow to rotate the ball. A lever is eccentrically connected to the pivot ball and extends upwardly within a closed, non-magnetic, housing. The upper end of the lever carries a magnet which actuates a switch outside the housing. An adjustable spring between the top of the lever and the housing permits adjustment of the sensitivity of the response.