**From:** ** **R.G.**
Date:** 6/22/2001 1:40 PM

Subject:

one Farad = one coulomb of charge stored per volt. A farad
is a huge amount of capacitance, so we'd like something smaller
- especially since making capacitors as big as 1F is very hard.

The techie world uses special words for certain units.

1/10th => "deci", as in deciliter, a tenth of a liter

1/100th => "centi", as in centimeter, one hundredth
of a meter

1/1000th => milli, as with our friend, the millimeter.

In the science world, we like to use three digits for precision
(that specifies things to the nearest 1/1000th) so it is handy
to use every third power of ten for units. Therefore, we use 1/1000,
1/1000000, and so on.

Historically, really big capcitors were about 1 micro farad, so
that got to being the generally used unit. Even today, we say
10K uF instead of 10 millifarad, which would normally be more
in line with the rest of the jargon.

one millionth => micro = 10**-6

one billionth => nano = 10**-9

one trillionth => pico = 10**-12

1/1000 of a trillionth => femto = 10**-15

A pico farad is literally a millionth of a microfarad, so it was
once called a mmF, for micro-micro-farad. Nano farads get a lot
more use in european electronics than in the US for some reason.
In the US, we tend to say "0.001uF" instead of 1nF.

4700pF is equal to 4.7nF, and 0.0047uF.

We do the same thing with resistors, just the other direction.
One ohm is a low resistance, so we use

kilo-ohms (1000 ohms) and megohms (one million ohms). Giga-ohms
and tera-ohms (10**9 and 10**12) values are not used much, largely
because they approach the resistance of air gaps. Switch designers
and transformer designers use milli-ohm (1/1000th ohm) numbers
for the resistance of things a lot.

R.G.