Two other reasons for running MM devices pneumatically rather than
electrically to be added to those that Robbie gave:
1. Some eras and some locations didn't have electricity;
2. The heat buildup from a bank of old-fashioned electromagnets can be
terrific, and the machines we are talking about would have to have them
packed in pretty tight, unlike the situation with larger pipe organs,
which could run on electrical actions.
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[ I know of a magnet-valve stack under a reproducing piano which was
[ completely surrounded with sound-absorbant, to muffle the noise of
[ the valves. Then a small fan had to be added to circulate the air.
[ -- Robbie