Franklin second to Microampere

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.

A CR2032 coin cell has about 225 mAh capacity. At 10 μA continuous draw, it lasts roughly 225,000 / 10 = 22,500 hours — about 2.5 years. At 1 μA, theoretical life exceeds 25 years, though self-discharge limits practical life to about 10 years.

Not from shock — the perception threshold is about 500 μA (0.5 mA) for DC and 1,000 μA for AC at 60 Hz. However, microampere currents applied directly to the heart (e.g., through a catheter) can cause ventricular fibrillation at as little as 50–100 μA, which is why medical device safety standards are so strict.

A glass pH electrode has an internal resistance of 10–1,000 megaohms. Drawing more than a few microamperes would cause voltage drops across this resistance, shifting the reading. Modern pH meters use high-input-impedance amplifiers that draw under 1 μA to avoid disturbing the electrochemical potential being measured.

Quiescent current (Iq) is what an IC draws when powered on but doing nothing — no signal processing, no load driving. For battery-powered designs, low Iq is critical. A voltage regulator with 1 μA Iq wastes far less standby power than one with 100 μA, directly extending battery life in always-on devices.

Pacemakers use constant-current output stages that regulate pulse amplitude to within ±5 μA. The pulse is typically 100–500 μA for 0.4–1.5 ms, just enough to depolarise heart tissue and trigger a contraction. Modern devices automatically adjust the current to the minimum needed, conserving the battery for its 8–12 year design life.