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 The Rod-Stone-Fortnight

How about a forty year old bit of British technological eccentricity?

I grew up in a world of imperial measures. At school, we were taught science partly in imperial units and partly using the CGS system (Centimetres, Grams, Seconds), but nobody used that in everyday life. But metrication was on the way. Although CGS used metric units, it is not the official 'Systeme Internationale' that everyone is now supposed to use. That is the MKS system (Metres, Kilograms, Seconds). When I was at university, everything was changing. I remember my text books on electromagnetism had lots of 'fudge factors' in all the equations. By inserting the appropriate value of the fudge factor, you could make the formula work in either MKS or CGS (EMU or ESU). Having separate units for electromagnetism and electrostatics (EMU = electromagnetic units and ESU = electrostatic units) seems a daft idea, but it made the formulae very simple (providing you stuck to one sort of problem, and didn't mix them).

When I started work, I discovered that metrication hadn't yet penetrated the aerospace industry, which was still using imperial units, but not as we had been taught in school. There we learned that mass was measured in pounds and force was measured in poundals. A force of 1 poundal would accelerate a mass of 1 pound at 1 foot/sec/sec. The weight of a 1 pound mass, ie the force exerted on it by gravity, was 32 poundals. In aero engineering, force was measured in pounds (ie pounds force, =32 poundals), and mass was measured in slugs (1 slug = 32 pounds). That preserved the relationship between force, mass and acceleration, ie 1 pound force applied to a slug mass accelerates it at 1 foot/sec/sec. Some of our colleagues worked on marine equipment, and at the time, oil tankers were getting quite large (but not as large as now). We realised that 16,000 tons was roughly a mega slug.

Against this background of metrication and confusion, one of my colleagues had the reactionary idea that we should more in the opposite direction, and adopt the Rod, Stone, Fortnight system. In case these units are not familiar to you. A rod is 5.5 yards (5.029m), a stone is 14 pounds (6.35kg) and a fortnight is 14 days. For good measure, he added a unit of volume, the noggin. A noggin is 1/2 pint (0.139 litre). We proceeded to calculate conversion factors and the values of various physical constants, using the RSF system.

Time 1 Fortnight = 1.21 * 10 ^6 seconds
Mass 1 Stone = 14 lb
Length 1 Rod = 16.5 ft
Volume 1 Noggin = 0.25 pint
  1 Cu Rod = 8.95 * 10^5 Noggins
Fuel consumption 10 Rods / Noggin - 1 Mile / Gallon
Power  2.875 Rod Stones / Fortnight = 1 micro horsepower
Speed 30 mph = 3.22 * 10^6 Rods / Fortnight
Gravity g = 2.85 * 10^12 Rods / Fortnight / Fortnight
Speed of light c = 7.15 * 10^14 Rods / Fortnight
Speed of sound Mach 1 = 7.9 * 10^7 Rods / Fortnight (at sea level)
Heat energy 1 ETU (English Thermal Unit) = 14 BTU
Mechanical equivalent of heat    J = 47.1 Rod Stones / ETU
Atmospheric pressure 1 Atmosphere = 4.12 * 10^4 Stones / Rod squared
Gravitational constant G = 4.875 * 10^-3 Rod cubed / Stone Fortnight squared

We were all serious engineers, but we had a sense of humour as well!

Centimetres are mad

To end on a serious note, I am equally happy using metric (SI) or imperial measurements, but I deplore the widespread use of centimetres by people who don't know any better (and some who ought to). Why do I think it is a bad thing to use centimetres? There are two reasons:

1 - The official reason : The SI (Systeme Internationale) is the international standard. It specifies that the preferred units smaller than a metre are the millimetre, the micro metre, the nanometre, etc, ie going down by a factor of 1000 each time. The decimetre (1/10 of a metre) and the centimetre (1/100 of a metre) are not preferred units.

2 - The practical reason : In the UK, imperial measures are used alongside metric units. It is quite easy to confuse a measurement in centimetres with one in inches. I have seen it done in engineering drawings. It would be almost impossible to confuse a measurement in millimetres with one in inches, because the numerical value would very different and so likely to be seen as suspect. The most famous case of an error of units was the one that caused the Mars probe to crash because someone had confused metric and imperial units.


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