The paper describes an experimental thermionic rheostat in which the spacing between the two thermionic cathodes is controlled by rotating the valve while one cathode is held stationary by means of an external permanent magnet. The valve has a small receiving-valve envelope and will control a.c. loads of up to 60 watts when operated as a series resistor from 240-volt mains. Three experimental closed-loop automatic-control systems are described whose electrical circuits consist simply of experimental valves actuated by mechanical error signals and controlling the current delivered to a.c. motors: the systems are a position control, a velocity-integral control for moving materials and an air-flow control.