APS News


"SI" Equals "System Imbecilic"

By Charles W. McCutchen (Overage physicist)

In the August/September issue, in a letter on the question of units, Ralph Tykodi wrote, "Publications of APS should follow International Recommendations on notations and terminology." Charles W. McCutchen has a different idea. -Ed.

Systeme Internationale units are the product of successive episodes of bloody-mindedness, a British term that means injuring others to demonstrate one's importance. The metric system got its start after the French Revolution. Before that there was anarchy in detail but agreement on principle. Different parts of Europe had different inches, but they were all about the length of a finger bone, the diameter of a crude broom stick. (The foot needs no explanation.) Units were chosen for ease of use.

To the French revolutionaries the units were part of the bad old days. Presto: enter the centimeter and decimeter, of which the inch is not far from the geometric mean. The new units were one-in-the-eye for royalty and everybody else. They were almost as far as possible from the units that people had chosen for themselves.

The meter is related to the Earth, but who cares? And if we did care, twice the polar diameter of the Earth is a billion inches to about a tenth of a percent.

Ordinary people kept using the units they liked. When I was in Stavanger in Norway in 1977 vegetables in the open-air market were priced by the half-kilogram, approximately one pound. English and American engineers likewise stayed with the familiar, making accommodations like the decifoot and kilopound where useful. Scientists and Continental European engineers, being more under the thumb of national academies, got stuck with the metric system. They did the best they could. Because the dyne/cm2 was ridiculously small, pressure got measured in kg/cm2 ~ 106 dynes/cm2 ~ 1 atm. The Angstrom was coined, 10-8 cm, a bit less than the spacing between atoms in a solid. It makes molecular structures easy to visualize.

Enter the reformers, overage physicists otherwise unemployable. They replaced cgs units by mks. As a young physicist I learned both, in reverse order. Both worked. Each had advantages for particular applications.

Though the metric system was awkward for most users it was convenient for some-how undemocratic!-until the next reformers struck. Which they did, with the fervor of Robespierre, the self-righteousness of Torquemada, the totalitarian ideal of conformity and the fascist weapon of compulsion. The metric system has all powers of ten available for use. What anarchy! People must be controlled, forced to use only those powers selected by the poo-bahs.

The new abomination is SI. Because the size of approved units progresses by thousands it is awkward for almost everybody. Democracy has been achieved. The Angstrom is verboten. One must use nanometers, which make molecular structures harder to think about. The Pascal (one apple-weight per desktop) is the approved unit of pressure, perfect in the eyes of the little Hitlers because it is unintuitive and unpopular. Here even scientists rebel. Many authors give pressures in atmospheres, thus using a familiar and enduring standard. Their papers will be understandable after the Pascal is forgotten-which it will be if scientists have any sense.

People who actually use units invent delightful and practical ones. As well as the Angstrom there is the ultracentrifugers' Svedberg, 10-13 cm (i.e. one Fermi/sec per g).
And they use happy coincidences like the density of water being about 2 slugs per cubic foot, thus the stagnation pressure in pounds/foot2 about equals the speed in feet/sec squared, a handy thing to remember if you are designing a fast boat.

If the height, in feet, of the center of gravity of a box-shaped boat above its center of buoyancy exceeds the square of the boat's width in feet divided by its draught in inches the boat will capsize. Run those numbers through your head before you put a tall load on your barge.

For airliner design and operation 100 kg, the mass of a passenger and baggage, is a convenient unit. Call it the pax. An airplane that carries 400 passengers and weighs 400,000 kg = 4,000 pax when loaded has a payload 10% of its gross weight. If, at the airport, 10 passengers get off, 10 pax of fuel can be added for the same total weight. Do these calculations in kg or tonnes and mistakes can crawl in.

We who do and make things have let the units dictators force us to use the metric system, and then keep us from using two thirds of that. We should throw off the yoke. We should use whatever units are convenient.

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Editor: Alan Chodos
Associate Editor: Jennifer Ouellette