An air valve designed for the purpose of triggering the expansion of a pneumatic cloud chamber utilizes Bridgman's ``unsupported area'' seal. The valve is controlled by a magnet constructed from the field coil of an inexpensive dynamic loudspeaker. This design allows vacuum tube control of forces up to 75 pounds. High speed of operation (∼1/300 second) recommends its use with counter control, and its extreme simplicity is advantageous in general work.