# What do old unit abbreviations like "mfd" and "kc" mean, and where did they come from?

I saw a recent answer that mentioned "mfd" as a unit of capacitance, and I can recall having seen that term in old schematics and ham radio articles. I've also seen other non-SI unit abbreviations, such as "kc" on old equipment and in old articles. The non-SI units don't stop there of course; I've seen "mcg" in medical contexts, and I've seen "kgf" used as a unit of force, and I'm sure there are many more, but please let's mostly confine the discussion to units used in amateur radio.

Would someone please explain older units and their abbreviations used in amateur radio such as "mfd" and "kc"? I would also welcome any relevant history that helps illuminate the topic.

• I don't think that there are others besides mfd and kc, although what I wrote probably applies to the inductance unit Henry (and subunits thereof) in a few cases. Hope you didn't mind me adding those tags; feel free to delete them. Jan 19 at 19:42
• I used to have a handy conversion chart to go from Mc/s to kHz and back, clipped from a radio magazine. Don't have much use for it now in the age of google. Jan 19 at 20:59
• I used to have a chart to convert Hz to cycles/sec and back. ;-) Jan 22 at 20:32

These were common abbreviations in use for Components, Radio equipment, and Printed publications in the 19th and most of the 20th century.

• Mfd = microfarad or μF
• kc = kilocycles or kHz. 1,000 cycles per second.
• μμf or mmf = pF (picofarad)

### Frequency

The change from kc to kHz and mc to MHz were done because someone though that Heinrich Hertz ought to be given credit for his work.

Heinrich Rudolf Hertz was a German physicist who first conclusively proved the existence of the electromagnetic waves predicted by James Clerk Maxwell's equations of electromagnetism. The unit of frequency, cycle per second, was named the "hertz" in his honor. ... The SI unit hertz (Hz) was established in his honor by the International Electrotechnical Commission in 1930 for frequency, ... It was adopted by the CGPM (Conférence générale des poids et mesures) in 1960, officially replacing the previous name, "cycles per second" (cps).

Many commercial and amateur radios were labeled kc or mc. My 1960s Collins S-Line manuals didn't use kHz or MHz.

### Capacitance

Informal and deprecated terminology, from Wikipedia:

The picofarad (pF) is sometimes colloquially pronounced as "puff" or "pic", as in "a ten-puff capacitor". Similarly, "mic" (pronounced "mike") is sometimes used informally to signify microfarads.

Nonstandard abbreviations were and are often used. Farad has been abbreviated "f", "fd", and "Fd". ... μ "micro" ... was also substituted with the similar-sounding "M" or "m", which can be confusing because M officially stands for 1,000,000 (or 1,000), and m preferably stands for 1/1000.

In texts prior to 1960, and on capacitor packages until more recently, "microfarad(s)" was abbreviated "mf" or "MFD" rather than the modern "µF". A 1940 Radio Shack catalog listed every capacitor's rating in "Mfd.", from 0.000005 Mfd. (5 pF) to 50 Mfd. (50 µF).

"Micromicrofarad" or "micro-microfarad" is an obsolete unit found in some older texts and labels, contains a nonstandard metric double prefix. It is exactly equivalent to a picofarad (pF). It is abbreviated μμF, uuF, or (confusingly) "mmf", "MMF", or "MMFD".

Summary of obsolete capacitance units: (upper/lower case variations aren't shown)

• µF (microfarad) = mf, mfd
• pF (picofarad) = mmf, mmfd, pfd, µµF
• And, formally, mc, being an abbreviation for megacycles, is wrong; it should be mc/s, which gets tedious. Hz means "cycles per second". Jan 21 at 13:54
• Or "Mc/s", to use the proper SI prefix. Jan 21 at 18:41
• Back in my youth (~1950s), mmfd was spoken as so-many mickey-mics. A 20 mmf compacitor (still called condensor by some back then) would be spoken as 20 mickey-mics. Jan 22 at 4:39
• @K7PEH -- and, to be absolutely clear, the "i" in "mics" is long, as in mickey-mikes. But it would be spelled mics. Jan 22 at 14:12
• @PeteNU9W -- But of course. Now, there is a local ham radio club here in the Seattle region called the Mike & Key club. Now, when I first heard this I wondered, "Who is this guy named Mike?". I would have expected the club to be named the "Mic & Key" club with the obvious assumption that ham operators would recognize the pronunciation of "Mic" being a sort of abbreviation for Microphone. Jan 22 at 20:02