Appendix A. Metric System of Unit Measures

Part Three: Appendixes

Appendix A. Metric System of Unit Measures

Scientists use a metric system of unit measures in their work. They use either the newer Système International (SI), which is the Meter-Kilogram-Second-Ampère (MKSA) system, or the older Gaussian system, which is the Centimeter-Gram-Second (CGS) system. The basis of both systems is metric and was created in France two centuries ago and refined, since then, to include electrical units. In this book, we use the SI of unit measures.

1. Basic Metric Units

The metric system's basic units were originally defined, as follows:

Length

The unit of length is the meter, m, which was defined as one ten-millionth of the great-circle distance from the North Pole through Paris, France and Barcelona, Spain to the equator.

Time

The unit of time is the second, s, which was defined as one 86,400^th (one [24 × 60 × 60]^th) of a mean sidereal day.

Mass

The unit of mass is the gram, g, which was defined as the mass of water at its densest filling a volume of one cubic centimeter. The SI unit of mass is the kilogram, kg.

Temperature

The unit of temperature is the kelvin, K, which was defined as one one-hundredth of the difference between the freezing and boiling temperatures of water under an atmospheric pressure that balances a 760-mm (29.92-inch) column of mercury.

Charge

The unit of electrical charge is the coulomb, C, which was defined as the number of electrons that pass in one second by a point on either of two, parallel conductive wires of indefinite length that are spaced one meter apart such that a magnetic force of 2 × 10^-7 newton, N, is applied between interfacing, one-meter lengths of the two wires. The coulomb contains about 6.2 × 10¹⁸ electrons.

The definitions of the newton and coulomb SI units establish the SI values of both the permittivity, ε₀, and permeability, µ₀, constants.

The following table lists the SI basic units of measure and their U. S. equivalents:

Basic Unit	Unit Code	SI Name	Si Code	U.S. Equivalent
Mass	M	kilogram	kg	2.2... pounds
Temperature	K	kelvin	K	1.8 fahrenheit
Charge	Q	coulomb	C	(same)
Length	L	meter	m	39.37.. inches
Time	T	second	s	(same)

2. Superunits and Subunits

Each superunit and subunit of a metric basic unit is, respectively, larger and smaller than the basic unit by a factor of a power of ten. A superunit's name begins with a Greek prefix, and, originally, a subunit's, with a Latin one. In either case, the name of a basic unit is appended to the prefix.

The prefixes and the basic units possess case-sensitive, one-letter abbreviations or codes. Therefore, in most cases, a metric unit is specified by no more than two characters. Metric codes do not end with periods. Plural codes do not use the letter, s. For example: (3 m) means 3 meters, but (3 ms) means 3 milliseconds, not 3 meters.

The following table lists the SI prefix names and codes for the positive and negative powers of ten:

Power of 10	Prefix Name	Prefix Code	English Meaning	Original Language
+18	exa	E	outside	Greek
+15	peta	P	spread	Greek
+12	tera	T	monster	Greek
+9	giga	G	giant	Greek
+6	mega	M	large	Greek
+3	kilo	k	thousand	Greek
+2	hecto	h	hundred	Greek
+1	deka	D	ten	Greek
- 1	deci	d	ten	Latin
- 2	centi	c	hundred	Latin
- 3	milli	m	thousand	Latin
- 6	micro	µ	small	Latin
- 9	nano	n	dwarf	Greek
- 12	pico	p	tiny	Italian
- 15	femto	f	fifteen	Old Norse
- 18	atto	a	eighteen	Old Norse

Notes:

1) These prefix codes are valid only when they are followed by a basic-unit code. Examples are: meter, m, versus millimeter, mm, and tesla, T, the SI unit of magnetic induction, versus terawatt, TW, a trillion watts (U.S.) or a billion watts (British).

2) This coding scheme respects the original metric conventions except for the following deviations: The Greek kilo- and hecto-prefix codes are in lowercase letters. The Latin micro-prefix code is the Greek-letter, µ, because the milli-prefix code uses the Latin-letter, m.

3) Niels Bohr, a Dane, is responsible for the addition of the femto- and atto-prefix names to the lexicon of the metric system.