Experiment 14: A Pulsing Glow
124 Chapter 3
BAckground
Maddened by measurement
Throughout most of this book, I’ve mostly used measurements in inches,
although sometimes I’ve digressed into the metric system, as when referring to
“5-mm LEDs.” This isn’t inconsistency on my part; it reflects the conflicted state
of the electronics industry, where you’ll find inches and millimeters both in daily
use, often in the very same data sheet.
The United States is the only major nation still using the old system of units
that originated in England. (The other two holdouts are Liberia and Myanmar,
according to the CIA’s World Factbook.) Still, the United States has led many
advances in electronics, especially the development of silicon chips, which have
contacts spaced 1/10 inch apart. These standards became firmly established,
and show no sign of disappearing.
To complicate matters further, even in the United States, you can encounter two
incompatible systems for expressing fractions of an inch. Drill bits, for instance,
are measured in multiples of 1/64 inch, while metal thicknesses may be mea-
sured in decimals such as 0.06 inch (which is approximately 1/16 inch).
The metric system is not necessarily more rational than the U.S. system. Origi-
nally, when the metric system was formally introduced in 1875, the meter was
defined as being 1/10,000,000 of the distance between the North Pole and the
equator, along a line passing through Paris—a quixotic, Francocentric conceit.
Since then, the meter has been redefined three times, in a series of efforts to
achieve greater accuracy in scientific applications.
As for the usefulness of a 10-based system, moving a decimal point is certainly
simpler than doing calculations in 64ths of an inch, but the only reason we
count in tens is because we happen to have evolved with that number of digits
on our hands. A 12-based system would really be more convenient, as numbers
would be evenly divisible by 2 and 3.
As we’re stuck with the whimsical aspects of length measurement, I’ve created
the charts in Figures 3-83 and 3-84 to assist you in going from one system to
another. From these you will see that when you need to drill a hole for a 5 mm
LED, a 3/16-inch drill bit is about right. (In fact, it results in a better, tighter fit
than if you drill an actual 5 mm hole.)