Lecture 3 – More and more about less and less

We left the measurement of length in Lecture 1 at the stage when the metric system was set up and based on the metre for length and the earth’s rotation for time. Just as the second is now defined in terms of atomic properties, so the metre has been re-defined as 650,763-73 wavelengths of the orange light emitted by the krypton 86 atom. Two of our basic standards are thus expressed in terms of atomic properties which will survive every natural catastrophe and which we believe to be the same in any part of the universe. Mass alone still has to be defined in terms of a material standard, the kilogramme.

It is possible to measure very short distances by light waves, and also by very simple devices such as the optical lever, Moire fringes, and the capacitance micrometer. Such measurements are particularly important in our newest industries, and if we can make them automatically we can have machine tools which are automatically controlled. We shall therefore look at some methods of automatically reading a scale and feeding the readings into a computer.

We shall also ask the question of whether, with our modern capacity for precise measurement, there is any limit to the smallness of things we can observe. We are already reaching the limits set by Brownian Motion – the agitation of the very atoms and molecules from which we and our observing instruments are made up. And we shall also find that when we try to observe anything, the act of observation may affect the object observed when we shine light to observe it, the pressure of the light on it may move it from where it was before we observed it.






RV Jones



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