Magnetic units, conversion factors

There are two systems of units commonly used in publications dealing with magnetic properties. These are the cgs-emu system and the International System of Units (SI). A number of usehil conversion factors are collected in Table 1. 

Sl-Sommerfeld units are used throughout this book, since it is essentially addressed to newcomers to magnetism. It is expected that the SI unit system will eventually be fully adopted by the magnetism research community. The definitions of the basic parameters and the conversion factors are given in Tables 4.6-4.S. 

Magnetic Symbol Dimensions SI unit CGS emu unit Exact conversion factors... 

Despite the fact that for the most part onl)t SI units are used in this book (see polic) on units), it is important to note some exact conversion factors between SI and CGS units since the remanence of obsolete CGS electromagnetic units are still important in the technical literature on magnetism, especiall) when examining data contained in old treatises or technical specifications. 

Unit conversion among magnetic properties depends on the dimensions used for the magnetic field, B. In the Gaussian and atomic units systems, electric fields and magnetic fields have the same dimensions, since the Lorentz force law is F (E -I- V X B/c). For these, Eq. [34] is valid for the conversion from atomic to Gaussian units. For S.I. units, where F = q(E -1- v x B), the conversion factor, F, is slightly different from Eq. [34] ... 

Equations (4.2) and (4.3) are given in the SI system of units, and consequently, caution should be exercised when using tables of magnetic susceptibilities. (The conversion factor between SI and CGS x units is 47T, i.e. Xgj = " Xqq5 )... 

In this book we have used two systems of units. The first is the SI system, which we use in the early chapters of the book and in particular for electromagnetic quantities. Factors of c therefore always represent the speed of light and never a conversion factor for magnetic units. The second is the Hartree atomic units system, defined by b = e = me = 1. In these units, c 137. Many physics texts use the system b = e = c = 1, since they are dealing with particles of different masses. Our concern is principally with the electron and chemistry, and the size of relativistic effects, which are measured by c, so Hartree atomic units are more appropriate. However, to keep the connection with SI units and to track quantities that involve the charge, the mass, or spin, the symbols ft, e, and m = me are retained in much of the development, whereas l/4 reo is usually omitted for clarity. 

In some cases conversion factors had to be used to give the tabulated parameters in the quoted units. These factors have been calculated with the fundamental constants given in 1. But the accuracy of the measured magnetic constants is not sufficient to differentiate between earlier used fundamental constants. 

Unit conversion factors for electric dipole-magnetic dipole polarizability... 

This contribution is sometimes referred to as the B term contribution to the Verdet constant. For molecules containing orbital degeneracies additional contributions arise (the A and C terms), in analogy with the closely related phenomenon - magnetic circular dichroism, discussed in O section Magnetic Circular Dichroism. For unit conversion factors, see O Table 11-9. 

Table 20.1 Magnetic Units and Conversion Factors for the SI and cgs-emu Systems...

Magnetic units may be a source of confusion because there are really two systems in common use. The ones used thus far are SI [rationalized MKS (meter-kilogram-second)] the others come from the cgs-emu (centimeter-gram-second-electromagnetic unit) system. The units for both systems as well as the appropriate conversion factors are given in Table 20.1. 

Because cgs and SI units are used in the literature, conversion factors between the two as well as definition of pertinent magnetism parameters along with their units are tabulated in Table 5.3. 

For free space, it is straight-forward to translate between CGS (Gaussian) and SI units, by substituting So —> 1/(477) and fio —> 4tt/c. For dielectric and magnetic materials, conversion between the two quantities is not so easy. For details, see a treatise on electromagnetism. An abbreviated set of conversion factors between CGS and SI units that are relevant to the quantities of interest in this text is given in Table D.2. The convenience of Gaussian units is illustrated by the fact that all quantities presented in Table D.2 can be related to the three fundamental units of the CGS system—cm, g, and s. 

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