CGS e.m. unit to EMU of current

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.

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. Both equal 10 amperes. The biot is the named unit; "EMU of current" is the generic label. It is like saying "SI unit of force" versus "newton" — same thing, different label. 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.