Wednesday 11 November 2015


One of the weirdly wonderful things about science is the naming of units.  It's a bit more limited than the naming of species or the naming of genes because there are fewer things that can be measured. I don't mean different ways of measuring the same fundamental thing: cubits, versts, ells, miles, metres, ångströms, fathoms, chains and parsecs all measure length.  It makes a difference as when two contributing engineering firms used newtons and pounds-force in the same space ship with costly results.

According the Système International d'Unités SI system there are 7 base units and you can measure everything else by combining these. Velocity is measured in m/s or ms-1 acceleration as ms-2 and so forth.  The seven fundamentals are illustrated in ring [R] are as follows
  • time second s
  • length meter m
  • mass kilogram kg
  • luminosity candela cd
  • molecular qnt mole mol
  • current ampere A
  • temperature kelvin K
You measure energy in Joules for convenience but that can be reduced to kg.m2s-2. Conversely the 'derived unit' for electrical resistance is the ohm which is kg.m2A-2s-3.  Conductance is the opposite/reciprocal and so is measured in mhos or siemens. The list is long with tributes to the great physicists, engineers and chemists of times past:
  • volt[a] for electromotive force; farad[ay] for electrical capacitance
  • henry for inductance; units kg⋅m2⋅s−2⋅A−2
  • hertz for frequency
  • pascal  for pressure [our weather-people give us the pressure in hectoPascals]
  • sievert, bequerel and gray for various aspects of radiation we're not meant to use Curies any more because One Ci is the same as 3.7 × 1010 Bq, so Curies are redundant and sooooo yesterday.
Being precise in the naming of parts is important in science, so that everyone is on the same page and we can make direct comparisons of things that are distant but too massive or delicate to shift so that they stand side-by-side.   There is a branch of elementary maths called dimensional analysis so you know what units are involved.  An engineer might want to trade off the stiffness of some material [in Newtons per metre kg⋅s−2] against its thermal conductivity [in watts per meter Kelvin m⋅kg⋅s−3⋅K−1] things cancel out so you're measuring in m⋅s−1⋅K−1. It was an inventory game that I was obsessively good at in school.  Words which in normal everyday speech are very similar in meaning, have precise definitions and differentiation in dimensional analysis:  Force, power and work for example:
  • Force: measured in newtons N = kg·m·s−2
  • Work: measured in joules J = kg·m2·s−2
  • Power: measured in watts W = kg·m2·s−3
Biology is a bit noisier and so much more interesting. The woolliness of living systems [and not just sheep] makes precision of measurement more difficult and it's even hard to establish the units in which to measure things.  Francis Galton, somewhat facetiously coined a millihelen for the amount of beauty that will launch a single ship. For vitamins and vaccines we are often reduced to IUs [international units] so that two messy preparations from different sources cane be compared as to their efficacy. The effectiveness of [artificial] insulin is determined by a "mouse convulsion test" which is quite as disturbing as it sounds - especially for the mouse.

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