CoachA conductor is a material that lets electricity travel through it. In the tester, a conductor bridging the gap closes the loop and lights the bulb.
Examples
Example 22.7 (Anatomy of a wire)
Look at the end of a lamp’s cable where it enters the plug: inside runs shiny copper — a conductor, the current’s road; around it wraps colored plastic — an insulator, the road’s fence. The copper does the work; the plastic keeps the current in and your fingers out. Every cable in your house pairs the two: a conductor inside an insulator.