DC Generators
In a commutator

None of the listed here
Mica and copper are equally hard
Copper is harder than mica
Mica is harder than copper

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DC Generators
When two D.C. generators are running in parallel an equalizer bar is used

To reduce the combined effect of armature reaction of both the machines
So that the two identical machines will pass approximately equal currents to the load
To increase the generated e.m.f.
To increase the series flux

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