I was also taught the same thing in school around 40 years ago +1 it seems that vacuum is the odd word out when placed in a lineup with (for example) continuum, individuum, menstruum, and residuum What's the difference between at hand, on hand and in hand At hand seems to me as if you have something in reach On hand is if you have something in stock And in hand can be used as if you have.
Is it necessary to put an article before the word "vacuum" If a 'vacuum cleaner cleaner' is a machine for cleaning vacuum cleaners, then the person who cleans the vacuum cleaner cleaner would be a 'vacuum cleaner cleaner cleaner'. Most of the she style labels i hear are half terms of endearment and half self mockery Likewise, objects with the label she are not necessarily unknown to the men involved. Considering their primary meanings, vacuum is used more often in a scientific context, in which case it means space completely or partially absent of any matter/air Electrical isolation amounts to using an air gap (or vacuum) as an insulating (nonconducting) medium
