DC Motor Interpole Polarity with respect to Field Polarity.
The "Mother-Daughter Rule" for Interpole Polarity.
Simply stated, the polarity of an interpole must be
the same as the preceding field pole, in the direction
of armature rotation. If you have ever witnessed
rapid, excessive build up of brush arcing, when applying
even small amounts of armature voltage,
this rule may have been broken.