Franklin second to Megaampere

Franklin (1706–1790) was the American polymath who proved lightning is electrical with his famous kite experiment in 1752. He introduced the convention of "positive" and "negative" charge that we still use today. He arbitrarily assigned positive to the charge on glass rubbed with silk — which turned out to be a deficit of electrons, giving us the unfortunate convention that current flows opposite to electron motion.

The roentgen (R) was defined in 1928 as the radiation exposure producing 1 ESU of charge (1 franklin ≈ 3.336 × 10⁻¹⁰ C) per cm³ of dry air at STP. This CGS-era definition stuck because radiation safety regulations were already built around it. Even though the SI gray replaced the roentgen for dosimetry, the roentgen — and its franklin-based definition — persists in US regulatory and medical imaging contexts.

The legacy unit of radiation exposure, the roentgen (R), is defined as the amount of X-ray or gamma radiation that produces 1 esu of charge (1 franklin) per cubic centimeter of dry air at STP. This definition dates from the 1920s when CGS-ESU was standard. Modern dosimetry uses grays and sieverts, but the roentgen and its franklin-based definition persist in some medical and regulatory contexts.

One Fr/s is about 0.33 nanoamperes — less current than a sleeping microcontroller draws. To equal the 1 A flowing through a phone charger cable, you would need about 3 billion franklins per second. The unit is spectacularly impractical for anything beyond electrostatics calculations.

Sort of. He labelled the charge on glass rubbed with silk as "positive," not knowing it was caused by removing electrons. When Thomson discovered the electron in 1897, it turned out electrons carry what Franklin called negative charge. So conventional current flows from + to −, opposite to actual electron flow. Engineers and physicists have lived with this "mistake" for over 250 years.

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.