The turbines and crankshafts of the mechanical engineers have proved to be very useful, as everyone knows. But they produce noise, and unfortunately need oiling every now and again. How much better if engines could be constructed whose moving parts have been eliminated! Solar cells, for the production of electricity, are one way to go: the result is an elegant 'engine' in which only the electrons are the moving parts. In a paper in Physical Review Letters, Reid, Ward and Swift describe another, but less familiar, transition to engines without moving parts. Moreover, by using a single gas both as the working part of the engine and as a heat-exchange medium, an old theoretical efficiency limit can be transgressed.
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