DC Generators
To avoid formation of grooves in the commutator of a D.C. machine

All of these
The brushes of opposite polarity should track each other
The brushes of same polarity should track each other
Brush position has no effect on the commutator grooving

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DC Generators
In a D.C. generator

Internal characteristic = magnetization characteristic ohmic drop
External resistance = internal characteristic armature reaction
Magnetization characteristic = external characteristic
External characteristic = magnetization characteristic ohmic drop armature reaction

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