Capacitance Converter (pF · nF · µF · mF · F)

In 2026, a junior PCB designer eyeballing a BOM that mixes "100n", "0u1", "4u7" and "3F" needs to ask whether all five values are physically realizable on the same board, and whether the magnitudes match the rails they decouple. The Metric Prefix Rack exists exactly for that moment: drop the BOM value in, see all five SI bins lit at once, and confirm with the application tag whether a 4.7 µF X7R or a 3 F EDLC is the right part. The rack is also pitched at students learning the SI system for the first time.

The earliest practical capacitor was the Leyden jar, demonstrated in 1745-1746 independently by Ewald Georg von Kleist in Pomerania and Pieter van Musschenbroek at the University of Leiden. The original glass-jar-with-foil device held perhaps 1 nF of capacitance at several kV — astonishing for the time and producing shocks vivid enough to be felt by a chain of 200 monks. The Leyden jar set the practical floor of capacitance technology at the nF level for over a century.

Michael Faraday at the Royal Institution in 1834 introduced the concept of "capacity" and the term "dielectric" — the medium between the plates. His work on what we now call relative permittivity (ε_r) explained why a Leyden jar with shellac between the foils stored more charge than one with air. The unit of capacitance — the farad — was named in his honour at the International Electrical Congress held during the 1881 Paris Exhibition. The committee chose the abbreviation F over Faraday's own preferred symbol.

The SI system formalized the prefix ladder in 1960 at the 11th CGPM in Paris, when the General Conference on Weights and Measures locked in the modern prefix names: pico (p, 10⁻¹²), nano (n, 10⁻⁹), micro (µ, 10⁻⁶), milli (m, 10⁻³). The farad itself had been derived in 1881; the milestone of 1960 was making the prefix-to-power-of-ten mapping internationally uniform. Before 1960, US schematics still used "MF" for microfarads — a confusing legacy that survived in vintage Fender guitar amplifier schematics into the 1970s.

The aluminium electrolytic capacitor, invented around 1875 by Charles Pollak and commercialised industrially in the 1920s, was the first device to push capacitance into the µF and mF range. By 1960 a 4700 µF / 50 V can was a standard part on TV-set PSU boards. The dielectric (a 0.01 µm aluminium oxide layer grown anodically) is the thinnest natural dielectric in any practical capacitor — which is why electrolytics achieve such high capacitance per volume but at modest voltage.

The supercapacitor or EDLC (Electric Double-Layer Capacitor) was patented by General Electric in 1957 and commercialised by Nippon Electric Company (NEC) in 1971. By 2026, leading-edge devices from Maxwell Technologies (now Tesla), Skeleton Technologies, and CAP-XX deliver 3000 F per cell at 2.85 V — pushing the once-theoretical farad into routine consumer use. The mF and F bins on the rack existed mostly as mathematical possibilities until the supercap commercialisation era; today they are real BOM lines on every EV and grid-scale energy buffer.

The IEC 60384 standard (Fixed Capacitors for Use in Electronic Equipment), first published in 1962 and now in its 7th edition, mandates that nameplate values use SI prefixes only — pF, nF, µF, mF, F — and codifies the tolerance letters (F=±1%, G=±2%, J=±5%, K=±10%, M=±20%) that appear on every modern silkscreen. The prefix rack matches this standard exactly so that a value read off an IEC-compliant BOM converts cleanly across the rack's five bins.