The correct answer is: A. Where low voltage and high currents are involved.
Copper brushes are used in DC machines to conduct current between the rotating armature and the stationary commutator. The brushes are made of a soft metal, such as copper, which is easily worn away. This allows the brushes to make good contact with the commutator, even as it rotates at high speeds.
The brushes are also held in place by springs, which ensure that they maintain good contact with the commutator. The springs are also designed to provide a certain amount of play, which allows the brushes to follow the contours of the commutator as it rotates.
Copper brushes are used in DC machines because they are a good conductor of electricity. They are also relatively inexpensive and easy to replace.
The other options are incorrect because:
- Option B is incorrect because copper brushes are not used in high voltage applications. In high voltage applications, brushes made of other materials, such as graphite, are used.
- Option C is incorrect because copper brushes are not used in both low voltage and high voltage applications. They are only used in low voltage applications.
- Option D is incorrect because copper brushes are used in DC machines.