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The instrument to measure current in amperes is called ammeter, while the instrument to measure charge in coulombs is called coulombmeter.

What happened to the -pere? Is there a historical reason for this?

Ampere-meter stands for the seldom used unit A*m.

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+1, intersting question, Jonas. I'm not aware of that, and it seems really strange. FWIW, French people say "‎ampèremètre" and Italian say "amperometro". I hope someone clarify this mistery. – user19148 Dec 15 '12 at 9:56
@Jasper sorry, I upvoted this question and after I did that there were 3 votes up here. However I do not have the same perception on this "strange downvotes" phenomenon, but - on the point - I think more people should use their quota of downvotes, because they make the site better. – user19148 Dec 15 '12 at 12:39
It would be nice, if the downvoters would leave a comment too. I have no idea how to improve now. – Jonas Stein Dec 15 '12 at 14:12
Another name for coulombmeter is voltameter, which sounds better IMO but could lead to confusion with voltmeter. – Edwin Ashworth Dec 15 '12 at 17:50
up vote 8 down vote accepted

Ampère, named after André-Marie Ampère, is first attested in the Oxford English Dictionary in the same year, 1881, as the term was adopted by the Paris Electric Congress. Ammeter appears a year later. In that short time, it rather looks as if the phonetical process of elision became reflected in the word’s spelling. Elision occurs when a sound is lost by the influence of those around it, and a consonant is particularly vulnerable when three occur together. In this case, the bilabial consonant /p/ seems to have been squeezed out by the repeated bilabial consonant /m/ that would otherwise have produced ampmeter.

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Most likely, it's derived from the common short form of the unit, amp (pronounced /ˈæmp/), which is obviously derived by shortening ampere. Then, in speech, the /p/ gets elided from the awkward /mpm/ sequence of *ampmeter, giving the current pronunciation.

(I don't know why the p was deleted in spelling, though. Perhaps it was filtered through a language like Italian that conventionally drops silent letters in spelling.)

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