No. 1860:
Mass, Length, and Time
Audio

Today, the albatross of pounds and inches. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

I'm bothered when I hear myself using English units in this program — feet, miles, Btu's. But how else can Americans, driving in traffic or getting dressed in the morning, understand me?

So, should I be using the Metric System? Well, not exactly. The Metric System wasn't much better than the English System, which you and I use. And it's obsolete; the outwardly similar International System of units has replaced it.

We need only a few basic units (like length, time, and mass) to write the equations of physics. Then the remaining units derive from this set. In the English and Metric systems, those basic units were all based on changeable features of the world around us.

The French first tried to define their meter as one ten-millionth of the distance from the equator to the North Pole. But that length varies slightly over time. We needed to base our measure of distance on something less changeable. Since 1980, the standard has been the distance that light travels during a certain short instant. No more
standard rulers or meter-sticks!

But, for the unit of mass, we do still use a standard one-kilogram object. It's a platinum-iridium cylinder, kept in a vault in Sèvres, France, with copies in other countries. That remains a problem because, when the various standards are gathered together and compared, tiny variances show up.

The English and Metric Systems were especially awkward in their energy units. The calorie and Btu were both based on the energy needed to heat water under certain conditions. Around 1850, we realized that heating and working are equivalent. After that, the unit of energy could be expressed as a force acting through a distance. No more calories or Btu's in the International System.

And, until 1967, we based time on the length of a year. But it turned out that each new circuit of Earth around the sun takes a fraction of a second longer. We finally adopted the cesium clock standard. Now a second is the time it takes for energy states of a cesium atom to make nine-point-however-many billion oscillations.

Today, our measurements all rest on this International System. Our English units are based upon it. The inch, for example, is defined as a 2.54-centimeter length. Yet we remain stuck with using the old units. We constantly struggle within an international marketplace — muddling about, trying to translate units.

It really is almost impossible to be bilingual in units. Canada and England have been more successful than we in changing over because they were fairly draconian: Road signs and the temperatures in weather reports, for example, changed overnight. Changing units is a lot like giving up cigarettes. Doing it gradually is lot harder than abruptly and painfully kicking the habit.

Each day we put off the change we lose ground in the global marketplace; and we make the inevitable more difficult.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)

You will find all kinds of explanatory material about the International System (or, more properly, SI — Système International) and other related issues on the web. An excellent print source is: F. Cardarelli, Scientific Unit Conversion: A Practical Guide to Metrication. (tr. M. J. Shields) New York: Springer, 1966.

For this episode, I leaned particularly heavily upon advice from my colleague Lewis Wheeler and my wife Carol Lienhard — for two very different reactions to this difficult subject.

Some definitions in SI units:

meter = distance that light travels in a vacuum during (1/299,792,459) seconds.
kilogram = (see text above)
second = the durations of 9,192,631,770 periods of radiation. corresponding to the transition between two specified levels of the ground state of the cesium 133 atom.
kelvin = (1/273.16) of the absolute temperature of the triple point of water. (the temperature in degrees Celsius is the temperature in Kelvin minus 273.16.)

And some of the many derived units include:

hertz = number of oscillations per second.
joule = energy, work, or heat expressed in newton-meters.
newton = force expressed as kilogram-meters/sec2.
watt = unit of power or energy in a unit time.
It is expressed in kilogram-meter2/second2.