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UNDER THE MICROSCOPE: The pithy remark that there is no such thing as a free lunch is a comment on charitable social work, but it also applies in the scientific realm. One of the oldest dreams of the inventor, and one that continues to fascinate many, is the concept of the perpetual motion machine, writes Dr William Reville

In essence, a perpetual motion machine is a device that, once started, will operate for ever, with no need of energy from the outside world. No such device has ever been built. Science declares one cannot be built, because the concept violates the laws of thermodynamics and a working model is therefore impossible. This does not keep inventors from trying - and patent offices regularly receive applications to patent new designs.