Gaussian electric current to EMU of current

In Gaussian units, electric and magnetic fields have the same dimensions, and Maxwell's equations look more symmetric — no ε₀ or μ₀ cluttering the formulas. When you study electromagnetic radiation in vacuum (starlight, cosmic rays, pulsar emissions), this symmetry is physically meaningful and simplifies calculations considerably.

Gaussian is a hybrid: it uses ESU conventions for electric quantities (charge, electric field, current) and EMU conventions for magnetic quantities (magnetic field, flux). This cherry-picking gives clean equations for both electrostatic and magnetic phenomena, at the cost of the speed of light appearing explicitly in equations linking electric and magnetic fields.

In SI, the fine-structure constant α = e²/(4πε₀ℏc) ≈ 1/137. In Gaussian units, ε₀ disappears and α simplifies to e²/(ℏc) — cleaner and more physically transparent. This is one reason particle physicists and quantum electrodynamics theorists favor Gaussian: fundamental constants combine more naturally, and the coupling strength of electromagnetism is immediately visible as α ≈ 1/137.

In Gaussian CGS, Maxwell's equations replace ε₀ and μ₀ with explicit factors of c, and the electric field E and magnetic field B end up with the same dimensions. The symmetric form ∇×E = −(1/c)∂B/∂t and ∇×B = (1/c)∂E/∂t reveals that E and B are equal partners in electromagnetic waves — a physical insight that SI's asymmetric constants obscure.

J.D. Jackson chose Gaussian units because they reveal the deep symmetry between electric and magnetic fields and make relativistic electrodynamics equations cleaner. His textbook, used in virtually every physics PhD program since 1962, cemented Gaussian as the "language" of theoretical electromagnetism. Later editions added SI appendices as a concession to modernity.

EMU stands for "electromagnetic unit." In the 1860s–1870s, physicists needed separate unit systems for electrostatic and electromagnetic phenomena because they had not yet unified them. The EMU system was built around magnetic force between currents, while the ESU system was built around Coulomb's electrostatic force. The ratio between them turned out to be the speed of light — a clue that led to Maxwell's equations.

Yes, exactly: the biot and EMU of current are two names for the same CGS-EMU unit. Under the traditional mapping used here, that unit corresponds to 10 amperes. The exact identity is between the two CGS names, not between the CGS unit and the post-2019 SI ampere. The CGS-EMU system also has named units for other quantities: the gauss (magnetic field), the oersted (magnetising field), and the maxwell (magnetic flux).

The EMU system was awkward for practical electrical engineering — 1 EMU of resistance (the abohm) equals 10⁻⁹ ohms, making everyday values absurdly large numbers. The SI system, adopted in 1960, unified mechanical and electrical units into one coherent framework with human-scale values. Practicality won over tradition.

Pre-1960 physics journals, particularly in geomagnetism, plasma physics, and early electrical standards work, routinely use EMU. Geophysicists measuring Earth's magnetic field historically reported results in CGS-EMU units (gauss, oersted, EMU). Some geophysics reference data still has not been converted to SI.

Weber and Kohlrausch discovered in 1856 that the ratio of the ESU to EMU charge was approximately 3×10¹⁰ cm/s — the speed of light. This was no coincidence: Maxwell showed that light is an electromagnetic wave, and the unit ratio reflects the fundamental coupling between electric and magnetic fields. One of the greatest insights in physics history, hidden in a unit conversion.