If we were to electrically charge two separated metal spheres (a typical Physics class prop): one with a positive charge, and the other with an equal negative charge, we could then perform a number of simple experiments.
For instance, we could connect the spheres together using a length of string. In this case, we would find that the charges collected on the two spheres were unaffected.
Next, we could connect the spheres using a copper wire. In this case, we would find that there was no charge remaining on either sphere.
Further investigation would reveal that charge must have flowed through the wire, from one sphere to the other, such that the positive charge on the first sphere is completely cancelled by the negative charge on the second, leaving zero charge on either sphere.
In this way substances could be classified into two main groups, depending on whether they allowed free flow of electric charge or not. Of course, this has been done for us by the scientists of the past, who have also named these two groups.
Conductors, which allow charge to pass freely through them, and Insulators, which do not.
From our experiments above, we have found that string is an insulator, and copper is a conductor.
As a general rule, substances which are good conductors of heat are also good conductors of electricity. Thus, all metals are conductors, whereas air, rubber, plastic, glass, and ceramic are insulators.
Metals are good conductors (both of heat and electricity) because at least one electron per atom is free, meaning it is not tied to any particular atom, but is, instead, able to move freely throughout the metal. Any external influence which moves one of them will cause a repulsion of other electrons which propagates through the conductor at near to the speed of light. It is this very property of free electrons flowing in a metal that forms the basis of modern electricity use.
In good insulators, such as rubber or plastic, all of the electrons are tightly bound to atoms (which are fixed), and so there are no free electrons, providing a high resistance to the flow of charge through the material. The properties of conductors and insulators are central pillars in the design of every electrical item we see today.
So who discovered this phenomenon?
The distinction between conductors and insulators was first made by the British scientist Stephen Gray in 1729, during his experiments with static electricity.
When creating a static charge on his glass tube, he noticed that the cork at the end of the tube (needed to keep moisture and dust out) generated an attractive force on small pieces of paper and chaff. When he extended the cork with a small fir stick plugged into the middle, the charge was evident at the end of the stick. So he tried longer sticks, and finally he added a length of thread connected with an ivory ball.
In the process he had discovered that the charge would carry over distance, and that the ivory ball would act to attract light objects as if it were the electrified glass tube. Soon he had tested his theories with the thread out to distances as far as 800 feet. In the process, Gray discovered the importance of insulating the thread ‘wire’ from contact with the earth, using silk (which is less of a conductor than the hemp thread they used as their main conductor).
They noticed that using wire supports to the thread leaked away the electrical charge, and discovered that electricity could carry around bends in the thread and that it appeared to be unaffected by gravity.