What is reduced when interpoles are used in a generator to overcome the armature reaction?

Using interpoles in a generator serves to mitigate the adverse effects of armature reaction, which can distort the magnetic field and lead to issues such as sparking at the brushes. Interpoles are placed between the main field poles and provide a corrective magnetic field that counteracts the distortion caused by the armature current. This results in a more stable and uniform magnetic environment, which reduces the tendency for sparking at the brushes during generator operation.

Sparking can occur when the commutator transitions occur under an uneven magnetic field, often caused by the armature reaction. By ensuring that the necessary adjustments are made to the field strength at the brushes, interpoles help improve commutation, leading to smoother and less disruptive electrical flow. This not only enhances the performance of the generator but also contributes to its longevity by minimizing wear and tear on the brush gear.

Thus, the use of interpoles significantly addresses the sparking issue, making it the correct answer in the context of the question.