I assume that whenever there's a /z/ sound after a /dʒ / sound, it's pronounced as /ɪz /. So, since we could never have 'is' contracted after /dʒ / in speech, nor do we have it in writing. Like we don't have 'My knowledge's more limited than yours', although this is not true for 'has', 'My knowledge's been lost'. Right?
Not an important question at all, though! 😀
You know, what I meant was that contraction originated from speaking, so, if we can't contract 'knowledge is' in speech, we can't do it in writing either.
Really? It seems to me 'knowledge is' can become 'knowledge's' alright. Is there a rule ?
Just because there is little or no discernable difference in pronunciation between knowledge is and knowledge's doesn't mean it isn't or can't be done. If you can say, "The car's parked outside." (and you can) I see no reason that you can't say, "Knowledge's useful." There might not be any practical value in doing so, but there's nothing wrong with it.