What is the best method for cleaning a dirty commutator?

The best method for cleaning a dirty commutator is the use of sandpaper. Sandpaper is specifically designed for smoothing and finishing materials, and when used carefully, it can effectively remove carbon buildup and grime from a commutator's surfaces without causing significant damage. The abrasiveness of sandpaper allows for precise control over the amount of material being removed, which is crucial in maintaining the integrity of the commutator.

While some other methods may clean the commutator, they may not be as effective or can risk damaging the surface. For example, using soap and water could clean off some dirt but may not effectively eliminate carbon deposits, and the moisture could potentially lead to corrosion. Emery cloth, while also abrasive, is typically more aggressive than sandpaper and might lead to over-polishing or scratching the surface. Kerosene can assist in dissolving grease but would not be effective for physically removing solid deposits like carbon. Thus, sandpaper stands out as the optimal choice for this task.