DC Generators
In a commutator

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

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

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

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