DC Generators
In a commutator

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

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

To increase the generated e.m.f.
To increase the series flux
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

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