U.S. Patent Specifications

13. In a vertical air speed responsive instrument, the combination of a chamber enclosing a substantial volume of air; a constricted passage connecting said chamber with the atmosphere; a differential pressure responsive member connected to respond to a difference in pressure inside said chamber and the atmosphere, respectively; a movable member connected to be actuated by said pressure responsive member; and an element responsive both to variations in pressure and temperature of the air inside said chamber and connected to increase and decrease the resistance of said passage to a flow of air therethrough upon a rise in pressure and drop in temperature, and a drop in pressure and rise in temperature, respectively, whereby said member will be moved in true response to a vertical air speed, irrespective of changes in altitude and viscosity of the air.