Section 28.4 Conduction of Electricity
Consider the experiment illustrated in Figure 28.4.1. Suspend an electrically charged ebonite rod that has been rubbed with a cat's fur. Fix a metal ball near one end of the suspended rod, and another metal ball a distance away. Now, connect the two metal balls by a metal wire. If you touch the far away metal ball, by another charged ebonite, then you will find that suspended ebonite rod swings away, demonstrating that electricity has traveled through the metal wire to the nearby metal ball, which exerts the electric force on the suspended charged ebonite.
Suppose, you repeat the experiment the two metal balls, with a thread of silk, wool or cotton, rather than a metal wire, you will not find any force on the suspended charged ebonite rod. But, if you were to repeat the experiment with cotton thread that has been moistened with a salty water, there will be a slowly rising force on the suspended rod. The ions in the salty water allow the charges to flow from one metal ball to the other.
These experiments help us classify all material objects according to their ability to conduct electricity. Metals and ionized water are considered to be good conductors, and silk, cotton, wool, and wood are called insulators or poor conductors.