Metrology as the science of measurement is one of the main challenges of the nano-scale. Referring to measurement needs and measurement devices, scientists and engineers often quote Lord Kelvin, Sir William Thomson: "In physical science the first essential step in the direction of learning any subject is to find principles of numerical reckoning and practical methods for measuring some quality connected with it. I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot meas- ure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of Science, whatever the matter may be." [PLA, vol. 1, "Electrical Units of Measurement", 1883-05-03].
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