Gaussian electric current to CGS e.m. unit

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.

The CGS e.m. unit of current (10 A) was inconveniently large for everyday lab work, while the CGS e.m. unit of resistance (the abohm, 10⁻⁹ Ω) was absurdly small. Physicists created "practical" units — the ampere, volt, and ohm — as decimal multiples that gave human-scale numbers. The ampere was set at 0.1 abampere. These practical units eventually became SI, while the "absolute" CGS units became historical footnotes.

In the 19th century, electricity and magnetism were treated as partially separate phenomena, leading to separate "natural" unit choices. The EMU system normalized magnetic permeability to 1; the ESU system normalized electric permittivity to 1; the Gaussian system mixed both. Once Maxwell unified electromagnetism, this fragmentation became unnecessary — but the systems persisted in literature for a century.

They introduced "practical" units — the ampere, volt, and ohm — as decimal multiples of CGS-EMU quantities. The ampere was defined as 0.1 abampere (CGS e.m. unit). This practical system eventually became SI, while the "absolute" CGS units faded. The factor of 10 was chosen for human-scale convenience.

The gauss (magnetic flux density, = 10⁻⁴ tesla) remains surprisingly common — refrigerator magnets are rated in gauss, and MRI field strengths are often quoted in both tesla and gauss. The oersted (magnetic field strength) appears in materials science. These CGS-EMU holdouts persist because their numerical values are more convenient for everyday magnets.

The SI was officially adopted in 1960, but the transition took decades. Most physics journals required SI by the 1970s, though astrophysics and plasma physics held onto Gaussian CGS into the 2000s. Some subfields never fully switched — you can still find new papers using gauss and oersted alongside tesla and A/m.