In order to keep the width of cloud‐chamber tracks down to less than 1 mm the time delay between passage of a ray and completion of an expansion must be 10−2sec. or less. When Geiger counter control is employed it may be difficult to obtain a sufficiently fast acting expansion control valve, particularly for a large cloud chamber. A valve combining a number of principles not all used by others has been constructed. By balancing pressures the movable valve parts can be made very light despite a large valve opening, and the controlling electromagnet can be quite small. Calculations were made concerning all the important design features. Even a small electromagnet must be laminated to insure sufficiently fast decay of its magnetic field. Most of the remaining time delay is caused by the aerodynamic resistance at the valve opening. For a valve diameter of 5.6 cm expansions are completed in 8×10−3sec. With a power consumption of 0.17 watt a small vacuum tube is sufficient for operation. By balancing pressures one could construct a much larger valve operating as fast as the valve described with only a slight increase in power consumption.