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Any of various devices used to measure the volume (and duration) of an audience's applause, and hence to gauge enthusiasm or popularity.
Originally and most notably an instrument devised by Hughie Green, Canadian-born television presenter, and popularized on his British television talent contest ‘Opportunity Knocks’, which ran from 1956 to 1978.
1970s; earliest use found in TV Times. From clap + -ometer.