Question

why do all the metal pieces get polished before the experiment?

Answer 1

because most of the metal react with oxygen from the environment to form metal oxide which leads to discontinity in the circuit so metal get polish before using it.

Answer 2

All the metal pieces get polished before any experiment because most of the metals are oxidize.

That might mean there is a thin coat of metal oxide on the outside of the metal.

It must be removed before performing any experiment. Some metals are polished by liquid and some are polished by sandpaper.