this had been bugging me for a while. roof-mushroom style attic fan
with junction-box style adjustable thermostat, looks pretty old and
like it had been maintained periodically by drunken chimps, wrong
sized sheet metal screws sticking out, etc; making me suspicious.
attic seemed overly warm on summer days. usual 1950s attic insulation,
i.e. less than currently recommended, meaning hot attic generating hot
house.

so, stuck a remote reading outdoor thermometer up there. gets up to
120+. seemed too high. took second look at fan setup. despite physical
appearance, seemed to be working OK every time I check on it. fan
capacity OK. twiddled around with thermostat settings, figured maybe
somebody had screwed up the calibration. remote thermometer still
showing 120+ up there on hot day. look at fan some more, it's running
OK every time I check. maybe thermostat is intermittent? finally wire
up boom box to thermostat and turn it up loud so i can tell from
downstairs when thermostat is opening and closing, with remote
thermometer to indicate temperature

answer comes very quickly when i come down from attic. turns out fan
is wired to attic light switch in hallway near attic hatch/ladder.
soon as i get down and turn the attic light off, music stops. turn
light on, music starts again. this, despite big clearly labeled (and
working) fan shutoff switch in attic.

haha.