One farad is so huge that the whole sky barely manages it
The farad, the unit of capacitance, is absurdly large for everyday electronics. Most circuit parts are rated in millionths or trillionths of one. By one calculation the entire ionosphere, the charged upper atmosphere hanging over the planet, holds about one farad relative to the ground, roughly what a single modern supercapacitor offers on a workbench.
Capacitance measures how much charge something stores per volt. A capacitor rates one farad if a coulomb of charge shifts the voltage between its plates by one volt, and the relationship is linear: halve the voltage and the stored charge halves too. The basic design, two conducting plates separated by an insulator, goes back to the Leyden jar of the eighteenth century.
Because the unit is so big, engineers live among its prefixes. Microfarads, nanofarads and picofarads cover most work, and millifarads are oddly shunned, so 0.0047 farads gets written as 4700 microfarads. Commercial parts run from about 0.1 picofarad to supercapacitors of 5000 farads. Below that smallest size, stray capacitance from wiring and circuit boards swamps the component; twisting two short insulated wires together gives about a picofarad anyway. Chip designers fret about femtofarads, and sensitive test gear can spot changes of tens of attofarads.
The workshop slang is colourful. Technicians call a picofarad a puff, as in a ten-puff capacitor, and a microfarad a mike. Before about 1960 microfarads were often marked mf or MFD, and a 1940 Radio Shack catalogue rated everything in Mfd, from 0.000005 up to 50. Older labels even carried the micromicrofarad, a clumsy double prefix meaning a picofarad. The reciprocal quantity, elastance, has an unofficial unit spelled backwards: the daraf.
The name honours Michael Faraday, who lived from 1791 to 1867. Latimer Clark and Charles Bright coined it in 1861, though at first for a quantity of charge. By 1873 it meant capacitance, and the International Congress of Electricians in Paris made that official in 1881.
Source: Farad
