CGS e.m. unit to Megaampere

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.

The Z Machine stores energy in massive capacitor banks (about 22 MJ) then discharges it through a converging array of transmission lines into a tiny central target in roughly 100 nanoseconds. The extremely short pulse duration means the instantaneous current reaches 26 MA, but only for microseconds. The peak power briefly exceeds 80 TW — more than the entire world's electrical grid.

At megaampere levels, the magnetic field generated by the current itself becomes an overwhelming force. In Z-pinch experiments, the current's own magnetic field crushes a metal cylinder inward at velocities exceeding 600 km/s, reaching pressures found inside giant planets. The material is compressed, heated to millions of degrees, and emits intense X-rays.

In a tokamak, the plasma current generates a poloidal magnetic field that, combined with external toroidal fields, creates the helical field geometry needed to confine plasma at 150 million degrees C. ITER needs 15 MA to maintain this confinement long enough for deuterium-tritium fusion to produce net energy.

The most extreme positive lightning superbolts — occurring over oceans and detected by satellite — may briefly reach 0.5–1 MA peak current. These are extraordinarily rare, representing perhaps 1 in 1,000,000 lightning strokes. A typical bolt is "only" 20–30 kA, about 50 times weaker.

Nobody puts a clamp meter around 26 MA. Instead, they use Rogowski coils (air-core toroids around the conductor) or B-dot probes that measure the rate of change of the magnetic field. The current is then calculated from Maxwell's equations. These sensors can respond in nanoseconds and survive the brutal electromagnetic environment.