CGS versus SI

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The SI or International System of Units versus the CGS or Centimetre–gram–second system of units is a constant source of confusion in the field of NMR. Both are based on the metric system, however the differences can result in quite different forms of the fundamental NMR equations. An example of this is Tesla versus Gauss units for the magnetic field strength.

CGS flavours

Note that in the CGS system, that there are different sets of units for electromagnetism. The major systems are ESU, EMU, Gaussian units, and Lorentz-Heaviside units. In NMR papers not using SI units, the EMU system is most commonly used.

Units of measurement

Quantity	Symbol	SI unit			CGS unit (EMU)			SI to CGS conversion factor
		Name	Symbol	Base units	Name	Symbol	Base units
length	L	metre	m	m	centimetre	cm	cm	10−2
mass	m	kilogram	kg	kg	gram	g	g	10−3
time	t	second	s	s	second	s	s	1
velocity	v	metre per second		m⋅s−1	centimetre per second		cm⋅s−1	10−2
acceleration	a	metre per second squared		m⋅s−2	galileo	Gal	cm⋅s−2	10−2
force	F	Newton	N	kg⋅m⋅s−2	dyne	dyn	g·cm·s−2	10−5
energy	E	Joule	J	kg⋅m2·s−2	erg	erg	g·cm2·s2	10−7
power	P	Watt	W	kg⋅m2·s−3	erg per second		g·cm2·s−3	10−7
magnetic dipole moment	μ	Newton-meters per Tesla		A·m2	electromagnetic unit	emu	abA·cm2	10−3
magnetic flux density (B-field)	B	Tesla	T	kg·s−2·A−1	Gauss	Gs	Mx·cm−2	10−4
magnetic flux density (H-field)	H	Amperes per metre		A·m−1	Oersted	Oe	g·cm·s−2	4π·10−3
magnetic flux	ΦB	Weber	Wb	kg·m2·s−2·A−1	Maxwell	Mx	Mx	10−8
magnetic susceptibility	χm			-			-	(4π)−1
electric current	I	ampere	A	A	abampere (or biot)	abA (or Bi)	abA (or Bi)	10−1
electric charge	Q	coulomb	C	A·s	abcoulomb	abC	abA·s	10−1

Constants

Constant	Symbol	SI unit			CGS unit (EMU)			SI to CGS conversion factor
		Definition	Value	Base units	Definition	Value	Base units
magnetic force constant	kA	µ0·(4π)−1	10−7	kg⋅m·s−2·A−2	Unity	1		4π·µ0−1
magnetic constant	µ0	µ0	4π·10−7	kg·m·s−2·A−2	Unity	1		µ0−1