Switch on and watch it run. Press Pause, then Next step to turn the coil 15° at a time and watch the current reverse as the coil passes the vertical. You can also grab a side of the coil (left picture) and turn it by hand. Then untick the commutator and try again.
Looking along the axle (from the commutator end)
- The two long sides of the coil carry current in opposite directions (⊙ out of the page, ⊗ into it).
- Fleming's left-hand rule pushes one side up and the other down, with the same-sized force F = BIL, always at right angles to the field. The pair of forces turns the coil.
- A stronger magnet (bigger B) is drawn with more field lines, closer together.
- The turning effect is biggest when the coil is horizontal, because the forces are then furthest from the axle (measured at right angles).
- It is zero when the coil is vertical, where both forces act in line with the axle.
- At the vertical position the gaps in the split-ring commutator X and Y are at the carbon brushes P and Q, so for a moment no current flows. The coil's inertia carries it past that point.
- Then the half-rings have swapped brushes, so the current in the coil reverses and the forces keep turning it the same way.
- Without the commutator, each end of the coil is joined to its own complete ring (a slip ring), so the current in the coil never reverses. The forces reverse after the vertical position, and the coil just rocks back and forth and stops.