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The direct conversion of heat into useful electrical work without utilization of moving mechanical parts has been successfully achieved in a novel device called the thermoelectron engine. This device is a heat engine in the thermodynamic sense because the working fluid, an electron gas, receives heat at a high temperature, rejects heat at a lower temperature, and delivers useful electrical work to an external load. This heat engine is also a thermionic device in that the electron gas is produced by emission from a hot cathode in a vacuum and by absorption or condensation of the electrons on a colder anode at a higher negative potential. The basic principle underlying this heat engine is that a calculable fraction of electrons emitted from a hot cathode possess sufficiently high values of emission velocity to overcome a retarding electrostatic potential barrier between cathode and anode in a vacuum. Thus these electrons can transform their high initial value of kinetic energy into useful potential energy at the colder anode; this potential energy can then be utilized by connecting cathode and anode externally through a matched impedance in the form of a load. Hence, this engine utilizes a selection process which results in a large value of output voltage per unit cell compared with the output of a thermoelectric generator per unit thermocouple.