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Recent developments in technology and the availability of cheap microprocessors have led to an increased interest in sensing devices, particularly so-called digital devices suitable for direct interfacing to computer systems. Unfortunately (or perhaps fortunately), the only thing at all digital about human beings is that most of us have ten fingers. We are analogue animals living in an analogue world. The quantities we need to measure are inherently analogue; they can in principle take a continuous range of values, though we may prefer to round the values to whole numbers at some stage. There is nothing very digital about a length or a temperature, and although matter is discrete it is certainly not so to our senses and not so to the vast majority of our sensors. In fact it is quite difficult to think of anything in nature that is inherently digital; almost the only example in measurement is in counting numbers of particles (photons, γ-rays, etc.).
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