Plural of clapper; the strikers inside bells that create sound when they strike the bell, or devices that make clapping sounds.
From 'clap' (striking sound) + '-er' (thing that does the action) + '-s' (plural). The term dates to Old English and describes the mechanical heart of bell-ringing.
Clappers had to be perfectly weighted and positioned—one poorly-designed clapper could render an expensive bell useless, so bell-makers were mathematical engineers solving physics problems centuries before modern acoustics.
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