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I've made an observation that some unstressed vowels seem to be pronounced more clearly in British English than in American. For example the second vowel in the word Venice in American seems to be pretty close to last vowel sound in the word ultimate whereas in British it seems to be pretty close to the vowel in is.
http://www.howjsay.com/index.php?word=venice
http://www.merriam-webster.com/dictionary/venice

Actually there seems to be quite a few words that, at least in the dictionaries, seem to be pronounced differently in this regard, for example the last vowel in the words accomplice, crevice and solstice and also the second vowel in the words ultimate, helicopter etc. How can I tell whether the vowel should be pronounced fully in a word when trying to emulate British English or when to reduce the vowel when emulating American English?

Also a bit more technical question: what is the above mentioned vowel? Is it the vowel indicated with the symbol ɪ or the Near-close near-front unrounded vowel? The vowel in the word bit.
http://en.wikipedia.org/wiki/Near-close_near-front_unrounded_vowel
Or is it possibly the vowel indicated with the symbol ɨ or the Close central unrounded vowel? The vowel that, according to Wikipedia, is used in the word roses in some dialects.
http://en.wikipedia.org/wiki/Close_central_unrounded_vowel
Comments  
Mr. Wookiee Is it the vowel indicated with the symbol ɪ

This may be the symbol in some systems, but the IPA representation is the schwa /Ə/. Most vowels that don't have at least secondary stress in AmEng do reduce to schwa.
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I think that many dialects have at least some features of what's called the "weak vowel merger", which is also known as the "Lennon-Lenin merger".

http://en.wikipedia.org/wiki/Phonological_history_of_English_high_front_vowels#Weak_vowel_merger

I guess that means that the distinction between /ɪ/ and the schwa is only important in stressed syllables. I don't know the exact quality of the unstressed reduced vowels for those who have the merger, but I think it can vary between [ɪ] and [ə], so it could be around [ɨ] or [ɪ̈] in the IPA chart.
Thanks Kooyeen, that was very helpful. I came across the article of Cicero and noticed that /ɨ/ is used in the pronunciation guide /ˈsɪsɨroʊ/
http://en.wikipedia.org/wiki/Cicero

And here's another article where /ɨ/ is used.

http://en.wikipedia.org/wiki/Tahmoh_Penikett

Let's say I want the broad transcript of the word Artemis (a figure of Greek mythology) would it be /ˈɑːrtəməs/, /ˈɑːrtəmɪs/ or /ˈɑːrtəmɨs/ or possibly something else?
Mr. WookieeLet's say I want the broad transcript of the word Artemis (a figure of Greek mythology) would it be /ˈɑːrtəməs/, /ˈɑːrtəmɪs/ or /ˈɑːrtəmɨs/ or possibly something else?
I don't know. Since only the first syllable is stressed, I guess that the schwa quality in the other syllables doesn't matter, and you could say that /ˈɑːrtəməs/ is the broadest transcription. I think that the way a speaker pronounces either of those schwas depends on the speaker's dialect. This is just my opinion though.
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Mr. WookieeLet's say I want the broad transcript of the word Artemis (a figure of Greek mythology) would it be /ˈɑːrtəməs/, /ˈɑːrtəmɪs/ or /ˈɑːrtəmɨs/ or possibly something else?
AmE: /ˈɑːrdəmɪs/

CJ