meter vs ammeter

meter and ammeter both are nouns.

Word	Noun	Adjective	Verb	Adverb
meter	Yes	No	Yes	No
ammeter	Yes	No	No	No

As nouns, ammeter is a hyponym of meter; that is, ammeter is a word with a more specific, narrower meaning than meter:

meter (noun)	ammeter (noun)
any of various measuring instruments for measuring a quantity	a meter that measures the flow of electrical current in amperes
rhythm as given by division into parts of equal duration
(prosody) the accent in a metrical foot of verse
the basic unit of length adopted under the Systeme International d'Unites (approximately 1.094 yards)

meter (verb)	ammeter (verb)
measure with a meter
stamp with a meter indicating the postage