When a unit of measurement is named after someone, convention has it that the word is written entirely in lower case when talking about that unit, but the single letter abbreviation of that unit is written in upper case.
Some examples should clarify this.
The unit of voltage is the volt, named after Volta, and abbreviated to V.
The unit of current is the ampere, named after Ampère, and abbreviated to A.
The unit of resistance is the ohm, named after Ohm, abbreviated in this case with the Greek letter Ω, pronounced omega, (not omeega).
So, 5V and 12 volts are both correct, but 5v and 12 Volts are not.
Additionally, these abbreviations are used in both the singular and the plural case; you don't add a letter s.
For instance, one volt is 1V, two volts is 2V, not 2Vs. One which catches many people out is the dB. One decibel is 1dB, three decibels is 3dB, not 3dBs.
The addition of a suffix to dB is used to state the reference to which the measurement or indication is made. For instance 3dBm is 3dB greater than 1mW. -4dBV is 4dB less than one volt. By inference, the incorrect 6dBs could lead the unknowledgeable to think it was 6dB greater than 1s (one second).
When a unit of measurement is not named after someone then the single letter abbreviation is not written in upper case.
Examples are the second, abbreviated to the lower case s, and the hour, abbreviated to h.
Furthermore there is no justification for the capitalisation of other frequently used abbreviations - a common mistake. (The only "justification" is ignorance.)
Examples are Va.c. and a.m. are correct, but VAC and AM are not. 10 seconds and 100ms are correct, 5S and 50mS are not.
Other conventions apply to prefixes which are used to denote magnitude or multipliers. A useful list can be found here , but a few common ones follow.
The lower case k for kilo, one thousand times, as in kg = one thousand grammes.
Lower case m for milli, the thousandth part, as in mA = the one thousandth part of an amp.
Upper case M, for mega, one million times, as in MHz = one million hertz.
This is something I saw in, I think, one of the electronics forums:
" Be careful. Despite more than 50 years to learn, capacitor manufacturers still get confused between mF (0.001 farad) and µF (0.000001 farad) and as a result, they label µF values as mF." Yes. Be very carefuil indeed.
These conventions would occasionally be explained in the monthly magazines, at least in the U.K., and some of them were explained to us in college. The absence of such explanations has lead to obvious confusion and the proliferation of every imaginable corruption of the correct way of writing them.
No one teaches them correctly, therefore no one learns them correctly.
We were taught in English Language lessons at school, that abbreviations, unless they were of Proper Names, or Proper Nouns, were not written in upper case and that each letter of the abbreviation was followed by a full stop. e.g. c.m.o.s., not CMOS.
Yes, I know what the OED, as they themselves abbreviate it (no stops), say CMOS, but the dictionary bows to common misuse instead of sticking to what they were (should have been) taught at school. In other words they are throwing in the towel.
It makes me wonder, sometimes, how the "education system" justifies employing English teachers.
Basic grammar and other linguistic rules which were taught us at school are casually and constantly brushed aside in deference to the latest illiterate herd.
Vis-à-vis the above, in my youth a cousin, after working at Thompson Boilers in Wolverhampton for some time, asked me if I knew the difference between a tube and a pipe.
My ignorance was enlightened. One of them has a seam, the other does not. It's as simple as that.
I don't remember which is which.
However, it seems that this basic, crucial, difference is no longer taught. I asked on a mechanical engineering forum. No-one had heard of it!
Ask an American and you will be astonished at the variety of incorrect answers. They talk about the ratio of outside to inside diameter, what they carry, and God knows what else.
Just as with our language, this basic definition has been effectively lost to the world.
There are so many things which are no longer taught and so many which are taught incorrectly.