sliding contact

I cannot understand the following sentence:

The efficiency of direct current machines is limited by the “brush drop” due to the resistance of the sliding contact.

Why not? Because of the grammar or the technical vocabulary?

‘sliding contact’

That is a machine part.

The full term is “sliding contact bearings”.

See … df/5_1.pdf

Hi, I am fairly certain that a sliding contact is different than a sliding contact bearing. The original sentence referred to electrical current and the voltage drop across the brushes of a dc motor, so the sliding contact here is an electrical connection that transfers electrical
current to a sliding or rotating part :