Kiloampere to Franklin second

A spot welder uses a large step-down transformer: high voltage at low current on the primary becomes very low voltage (1–2 V) at enormous current (5–30 kA) on the secondary. The copper electrode tips concentrate this current into a small spot, melting the metal in milliseconds. Total power is only 10–60 kW — it is the concentration that does the work.

A typical 14 AWG house wire rated for 15 A would vaporise almost instantly at 1 kA — the I²R heating would melt copper in milliseconds. Industrial busbars carrying kiloamperes are massive rectangular copper or aluminum bars, sometimes water-cooled, with cross-sections of 10–100 cm² to keep current density manageable.

A typical negative cloud-to-ground stroke peaks at 20–30 kA for about 1–2 microseconds. Positive lightning (rarer, about 5% of strikes) can exceed 300 kA. The total charge transferred is only 1–5 coulombs because the pulse is so brief — enormous current, tiny duration.

Aluminum oxide dissolved in molten cryolite at 960 degrees C requires direct electrolytic reduction to separate aluminum metal. Each smelting pot runs at 4–5 V but needs 150–500 kA because the electrochemical reaction requires massive charge transfer. A single smelter may consume 1–2 GW — as much as a small city.

Circuit breakers rated for 10–200 kA interrupting capacity use arc-quenching chambers to extinguish the plasma arc that forms when contacts open under fault current. High-rupture-capacity (HRC) fuses have sand-filled ceramic bodies that absorb the arc energy. Without these devices, a short circuit on a utility feed would weld everything in the panel into slag.

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.