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A theory of electrostatic attraction, influenced by field emission, is proposed to explain the Johnsen-Rahbek effect between the flat and polished surfaces of a metal and a semi-conductor in contact. The application and advantages of the effect as a clutch are described, and the results of a large number of life tests are summarized. It is concluded that the unreliability of such a clutch, due mainly to different amounts of wear, is caused by relatively small differences in the polished surfaces. Consequently it is suggested that the electrostatic clutch is not, in general, a practical proposition, but more likely applications of the effect lie in relay devices and valves for controlling gas flow.