The International System of Units, informally known as the metric system — the way in which the
world measures everything from coffee to the cosmos — will change in a way that is more profound
than anything since its establishment following the French Revolution. •
It will be a turning point for humanity.
Every day, scientists and engineers use measurements in design and research, but most of us give little thought to the basic definitions of our measurement
system, and fewer still know that in the next year those
definitions are in for some major changes. Take the volt, for
Readers of this magazine ought to know the definition
of the volt, but I’ll bet that most don’t. I graduated with
three engineering degrees and at the time, I could not have
told you the definition of the volt. Last year, at my 50th
college reunion, I gave a seminar in the EE Department on
electrical metrology. Most of the EE professors and many
students were present. I asked, “Who knows the definition
of the volt?” … Silence.
So, let’s talk about the volt. The volt is a derived unit
which means that once the kilogram, meter, second, and
ampere are defined, the equivalence of mechanical and
electrical power dictates the definition of the volt. See the
formal definition below.
The creation of the metric system in 1790 was the first
time that scientists had the idea that measurement units
should be connected with constants of nature. At the time,
the Earth itself was seen as the best constant of nature. So,
the second was defined as 1/86400 of a day, the meter
was a best estimate of 10-7 of the distance from the equator
to the north pole, and the kilogram was taken as 10-3 of the
weight of a cubic meter of water.
Since neither the surface of the Earth or a box of water
of the Kilogram
as We Know It
This November, in Versailles, France, representatives from 57 countries are
expected to make history. They will vote to dramatically transform the international
system that underpins global science and trade. This single action will finally realize
scientists’ 150 year dream of a measurement system based entirely on fundamental
properties of nature.
By C. A. Hamilton
One volt is defined as the difference in electric potential between two
points on a conducting wire when an electric current of one ampere
dissipates one watt of power between those points.
Photo Credit: J.L. Lee/NIST
76 September/October 2018