Physics & Society

International System of Units & Trueness

Referred to as the SI, covering the base units (metre, kilogram, second, kelvin, candela, ampere, mole) and the derived units.

Provides a basis for ensuring that reliable measurements give the same answer wherever they are made in modern society.

This system is based on and, in turn, supports continual and long-term research in fundamental science and technology.

There are many and distinguished examples where fundamental physics has contributed to the development of the SI and traceable measurements, involving several Nobel prizes in Physics.

Accurate measurement: Is it the domain of the Engineer or the Physicist?

“The number of electrical measuring instruments recently devised is very great. The practical man is not satisfied with the delicate instruments of the physicist, whilst the latter, of course, cannot be satisfied with the results of the measuring instruments arranged by engineers and technical electricians, however satisfactory for industrial purposes” (The Telegraphic Journal 1884, quoted in [G. Gooday,“The values of precision”, ed. M.Norton Wise, Princeton University Press.])

A broad generalisation:

  • task of the physicist to provide best estimates of the true value of a physical quantity. Here “truth” (trueness) refers not simply to a freedom from error, but to something rather more absolute
  • achieving good precision is the task of the measurement engineer

Comparing different measurements, even of different quantities

If traceability is provided in an unbroken chain of comparisons to the SI, then not only can the results of measurements of a particular quantity be compared, but also measurements of different quantities.

The key to this observation is the interesting distinction between indirect measurements where:

  • the formula used to calculate the measurement result is an expression of a physical law, e.g., F = ma (Newton’s second law, if the mass, m, is constant) relating different quantities
  • only an empirical “recipe” is used, of local validity but only limited universality.



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