Maggie's Farm

The situation regarding the metric system is far more complicated than the Slate article indicates. And it has nothing to due with how we price bread or gasoline. If it were only that, life would be so much easier.

The US was one of the original signers of the metric convention in the Nineteenth Century. As a consequence, all customary American units are defined by law in terms of metric standards. For example, 1 foot is defined to be 0.3048 meters (exactly).

But any engineering student from the not so distant pass will know (and will have learned) that there are are several systems of units, both metric and customary, all of which are in use.

The customary American units are the pound-force, pound-mass, second, foot, British thermal unit, etc. In this gravitational system, F = ma/gc. The factor gc is a units conversion so that 1 lbm weighs 1 lbf are the Earth's surface. This was the source of NASA's troubles on the Mars probes. All electrical units have always been fully metric.

But American engineers also encounter the pound-force, slug, etc units, and the poundal, pound-mass etc units. For the most part, these are academic systems and have never replaced the customary system, which is still in use. In these dynamic systems, F = ma.

The original metric system was the centimeter, gram, dyne, second, erg, calorie system plus a mishmash of inconsistent electrical and magnetic units. This mishmash has driven all so-called reforms since, and the changes are not yet played out. The cgs system is still in use by doctors, chemists and biologists because it is convenient to their work. In the cgs system, F = ma.

When I was an engineering student, the cgs system had been replaced by the meter, kilogram, Newton, second, Joule system, and some rationalization of the electromagnetic units had been achieved. It was widely used by physicists, and ignored by all sensible people. In mks, F = ma.

Then, while was an engineering faculty member, SI (System International) arrived. There was hardly any change from the mks system except for the final (?) rationalization of the electromagnetic and now nuclear radiation units. In SI, F = ma.

But that is not the whole story. European engineers, like their American counter parts, don't like SI (or cgs or mks), and they often use (unofficially and discountenanced) the kilogram-force, kilogram-mass, meter, second, etc., system. In this system, F = ma/gc.

Practicing engineers have to deal will stuff that was designed and built years, decades and even centuries ago, and to do so they must know (or find out) what system of units their predecessors used.

So, each modernization of units creates another layer of meaning that must be translated into earlier layers of meaning, with all the possibilities for stupidity and error that humans can devise.

The SI clowns are not done, and redefinitions of the second, meter have been achieved and soon the kilogram will be redefined. However, G_d willing, Americans will always be able to buy bread by the pound (mass or force) and gasoline by the gallon, and pay for them in US dollars.