A loop that can
never stop turning
Opposite arms carry current opposite ways, so the field lifts one and presses the other. That pair is a couple, and a couple is pure rotation — until half a turn later, when it would start braking instead.
lawτ = NIAB sinθ, with θ between the loop normal and the field. Maximum when the loop lies along the field, zero when it faces it.
trySwitch the commutator off and let it run.
noticeIt overshoots, the torque reverses and it rocks to a stop. With the commutator the current flips at that instant instead.
in lifeCordless drills, fans and window motors all still run on brushed commutators.
found byFaraday demonstrated continuous rotation in 1821; Sturgeon's commutator of 1832 made it practical.