# Conversion Tables for Units Chapter 1
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1.6 Conversion tables for units
The table below gives conversion factors from a variety of units to the corresponding SI unit.
Examples of the use of this table have already been given in the preceding section. For each
physical quantity the name is given, followed by the recommended symbol(s). The SI unit is
given, followed by the esu, emu, Gaussian unit (Gau), atomic unit (au), and other units in
common use, with their conversion factors to SI. The constant ζ which occurs in some of the
electromagnetic conversiton factors is the (exact) pure number 2.997 924 58
×
10
10
= c
0
/(cm s
-1
).
The inclusion of non-SI units in this table should not be taken to imply that their use is to be
encouraged. With some exceptions, SI units are always to be preferred to non-SI units.
However, since may of the units below are to be found in the scientific literature, it is
convenient to tabulate their relation to the SI.
For convenience units in the esu and Gaussian systems are quoted in terms of the four
dimensions length, mass, time, and electric charge, by including the franklin (Fr) as an
abbreviation for the electrostatic unit of charge and 4
π
ε
0
as a constant with dimensions
(charge)
2
/(energy
×
length). This gives each physical quantity the same dimensions in all
systems, so that all conversion factors are pure numbers. The factors 4
π
ε
0
and the Fr may be
eliminated by writing Fr = esu of charge = erg
½
cm
½
= cm
3/2
g
½
s
-1
, and 4
π
ε
0
= ε
ir)
= 1 Fr
2
erg
-1
cm
-1
= 1, to recover esu expressions in terms of three base units (see section 7.3 below). The
symbol Fr should be regarded as a compact representation of (esu of charge).
Conversion factors are either given exactly (when the = sign is used), or they are given to the
approximation that the corresponding physical constants are known (when the
sign is used). In
the latter case the uncertainty is always less than ±5 in the last digit quoted.
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