# Old capacitor markings (M) letter - actual capacitance

I am repairing an old tube audio gear from Japan. There are several old oil capacitors, some of them are labelled .01 (M) or .033 (M). Is it 0.01 and 0.033 μF?

There are other capacitors from the same manufacturer and they are clearly labelled with either pF or μF. I do not have a possibility to measure the capacitance. Also, I do not have the schematic.

It looks like this:

M is also used to indicate +/- 20 percent tolerance and this is the most likely meaning in this case. In this era a fractional value always meant microfarad. (Curd seems to be a prankster.)

• I'm not a prankster (at least not in this case ;-) apart from that I think actually that your answer is correct +1
– Curd
Oct 24, 2019 at 8:34

yes, M was traditionally used for micro on capacitors.

• Could you please add some reference or proof for that claim? Not that I doubt your answer, but some more information (e.g. pics, samples or explanations) would be convenient. Oct 23, 2019 at 7:50

Note, that in old times unit for measuring capacity was length, i.e. m (meter) or cm.

It meant the capacity of a spherical capacitor of the radius $$\R\$$ (one plate is thought as a conducting sphere of radius $$\R\$$; the other plate is far away (i.e. at infity)).
Its capacity is proportional to its radius:

$$\C=4\pi{\epsilon}_0 R\$$

1cm $$\\approx\$$ 1.11265pF,
1m $$\\approx\$$ 111.265pF

Though .01m $$\\approx\$$ 1pF seems to be a too small value for the capacitor shown in your picture.

EDIT:

This was meant as serious answer. See e.g. following references:

After plausibility checking the resulting capacity value I don't think, however, that this fact applies to the given component: the spatial size of the component is much to big; and 1pF is a capacity value much too small for such a component (see my remark given in the original answer above).
Also the fact that it is an upper case M and printed in parenthesis can be a hint that it doesn't mean physical unit "meter".

I guess actually the unit is µF (without any hints printed on the capacitor; so it is a 0.01µF = 10nF capacitor) and "(M)" indicates the tolerance (as EinarA) noted.

• Do you have a source for this? I've never heard this before and would like to learn more about it. Oct 23, 2019 at 23:11
• Read any old electronics text from 1930 or before... Read Tesla's diary, he uses nothing but meters. Oct 24, 2019 at 0:35
• I will say, that capacitor is probably from the 60s-70s, where "m" commonly meant micro. Oct 24, 2019 at 0:36
• @Hearth: see the references given in the edited answer
– Curd
Oct 24, 2019 at 7:33

M=mega p=pico

I'm.earlier times capacity notations were expressed in terms of picofarads (p).

In 0.01M:

$$0.01 Mf = M01 = 0.01 Megapicofarad = 0.01\cdot10^6\cdot10^{-12}=0.01\cdot10^-6=0.01μF =10nF$$

• Do you have a source for this? I don't think that's quite the way it was intended to be read, though it does happen to work out the same as M being used as a stand-in for μ on capacitors of this era, which is what I've always heard. May 9, 2021 at 13:30