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,
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
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
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
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
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)
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
joule = energy, work, or heat
expressed in newton-meters.
newton = force expressed as
watt = unit of power or energy in
a unit time.
It is expressed in
The Engines of Our Ingenuity is
Copyright © 1988-2003 by John H.