Faraday constant

Michael Faraday, a bookbinder's apprentice who became the greatest experimentalist of the nineteenth century, established in 1834 that the mass deposited in electrolysis is proportional to the charge passed and to the equivalent weight of the substance. He had no idea that charge came in lumps — the electron was sixty years away — but his laws only make sense if it does, and the constant named after him is exactly that lump multiplied by Avogadro's number.

It is the conversion factor of electrochemistry. Depositing one mole of copper from Cu²⁺ takes 2F = 192,971 C, which at 10 A is five and a half hours. It sets the scale of the Nernst equation through RT/F ≈ 25.7 mV at room temperature, it converts battery capacity in ampere-hours into moles of active material, and it fixes the theoretical energy of a fuel cell. Since 2019 both N_A and e are exact by definition, so F is exact too — a rare case of a chemistry constant with no error bar at all.