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#### 8.1.2 CGS Units Systems

The ESU system derives the electromagnetic units from its unit of charge, the statcoulomb, which is defined from Coulomb’s law. The statcoulomb equals $${\rm dyne}^{1/2}\,{\rm cm}$$ or $${\rm cm}^{3/2}\,{\rm g}^{1/2}\,{\rm s}^{-1}$$. The unit of current, the statampere, is statcoulomb sec, analogous to the relationship in SI. Other electrical units are then derived in a manner similar to that for SI units; the units use the SI names prefixed by ‘stat-’, e.g., ‘statvolt’ or ‘statV’. The prefix ‘st-’ is also recognized (e.g., ‘stV’).

The EMU system derives the electromagnetic units from its unit of current, the abampere, which is defined in terms of Ampere’s law. The abampere is equal to $${\rm dyne}^{1/2}$$ or $${\rm cm}^{1/2}{\rm g}^{1/2}{\rm s}^{-1}$$. The unit of charge, the abcoulomb, is abampere sec, again analogous to the SI relationship. Other electrical units are then derived in a manner similar to that for SI units; the units use the SI names prefixed by ‘ab-’, e.g., ‘abvolt’ or ‘abV’. The magnetic field units include the gauss, the oersted and the maxwell.

The Gaussian units system, which was also known as the Symmetric System, uses the same charge and current units as the ESU system (e.g., ‘statC’, ‘statA’); it differs by defining the magnetic field so that it has the same units as the electric field. The resulting magnetic field units are the same ones used in the EMU system: the gauss, the oersted and the maxwell.

The Heaviside–Lorentz system appears to lack named units. We define five basic units, ‘hlu_charge’, ‘hlu_current’, ‘hlu_volt’, ‘hlu_efield’ and ‘hlu_bfield’ for conversions with this system. It is important to remember that with all of the CGS systems, the units may look the same but mean something different. The HLU system and Gaussian systems both measure magnetic field using the same CGS dimensions, but the amount of magnetic field with the same units is different in the two systems.

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