Some substances readily allow the passage of electricity through them, others do not. Those which allow electricity to pass through them easily are called conductors. They have electric charges (electrons) that are comparatively free to move inside the material. Metals, human and animal bodies, and the earth are conductors. Most of the non-metals like glass, porcelain, plastic, nylon, and wood offer high resistance to the passage of electricity through them. They are called insulators. Most substances fall into one of the two classes stated above*.
When some charge is transferred to a conductor, it readily gets distributed over the entire surface of the conductor. In contrast, if some charge is put on an insulator, it stays at the same place. You will learn why this happens in the next chapter.
This property of the materials tells you why a nylon or plastic comb gets electrified on combing dry hair or on rubbing, but a metal article like a spoon does not. The charges on metal leak through our body to the ground as both are conductors of electricity. However, if a metal rod with a wooden or plastic handle is rubbed without touching its metal part, it shows signs of charging.
*There is a third category called semiconductors, which offer resistance to the movement of charges which is intermediate between the conductors and insulators.
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