CGS e.s. unit to CGS e.m. unit

The e.m. unit equals 10 A while the e.s. unit equals 3.3 × 10⁻¹⁰ A — a ratio of about 3 × 10¹⁰, which is the speed of light in cm/s. This enormous factor reflects the fundamental relationship c² = 1/(ε₀μ₀). The two systems were designed to simplify different sets of equations, and the speed of light is the price of bridging them.

In vacuum tubes and cathode ray experiments, electrostatic forces dominate — no magnetic materials, no currents in bulk conductors. The ESU system made Coulomb's law beautifully simple: F = q₁q₂/r² with no constants. For computing electron trajectories in early TV tubes and oscilloscopes, this simplicity was genuinely helpful.

Early cathode ray tubes used electrostatic deflection plates to steer the electron beam. Engineers working in CGS-ESU could calculate beam deflection angles directly from plate voltage and geometry using Coulomb's law without extra constants. The tiny ESU currents matched the actual beam currents (microamperes), making the numbers more intuitive than working in amperes for these minuscule electron flows.

Check the context and the magnitude of numbers. If currents are tiny numbers where you would expect amperes, it is ESU. If they are 1/10 of expected ampere values, it is EMU. Good papers state which system they use, but many older ones do not. The equations themselves also differ — look for factors of c or 4π.

Technically yes, but clumsily. In pure CGS-ESU, the magnetic field has dimensions involving the speed of light, and equations for inductance and magnetic force become awkward. This is exactly why the Gaussian hybrid was invented — it uses ESU for electric quantities and EMU for magnetic ones, giving clean equations for both.

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.