Why did you want the metals to be clean or polished before you tested them for electrical conductivity?

Answer 1

Many metals combine with oxygen in the air, forming a layer of metal oxides on the surface. Many metal oxides have different conductivity than the pure metal, and some are essentially not conductive at all; aluminum oxide would be a good example of an extremely good insulator.

Some oxide/metal junctions can even form simple diodes! Since you're interested in just the conductivity of the pure metal, you have to remove the oxides first. For really accurate measurements, this is generally going to take place in a noble gas blanket such as argon, so that the oxygen in the air can't get to it to form more oxides. Going back to the example of an aluminum surface, if you polished it in air, it would form a new oxide layer far faster than you could buff it off.

Answer 2

You should clean metals before trying an experiment to ensure that nothing an accounted for is introduced into the experiments. Chemicals and other substance, if left on your tools, can affect the outcome of your experiment.

Answer 3

Cleaning or polishing the metals before testing them for electrical conductivity removes any dirt, oxide layers, or other contaminants that could interfere with the conductivity measurements. This ensures that the results of the test are accurate and not affected by external factors.