Text: Conductors and insulators

2. The property of electrical conduction can be illustrated by an experiment. One end of a long thin copper wire is connected to an electroscope and the other end to a small brass knob mounted on a glass pedestal. When a charged rubber rod is touched to the knob, the gold leaf of the distant electroscope rises immediately. Electrons have been conducted along the wire. If a positively charged rod contacts the knob, electrons flow away from the electroscope, leaving the gold leaf with a positive charge.

3. If the copper wire in the above experiment is replaced by a non conductor, like a silk thread, the electroscope cannot be charged by the rod contacting the distant knob. Poor conductors, such a glass or amber, are used to support metal parts of electrical apparatuses for them to be insulated from unnecessary losses of electricity. For an electroscope to retain its electric charge the gold leaf and stem are insulated from the electroscope case with amber.

4. The difference between a conductor and insulator, or dielectric, is that in a conductor there are free electrons, whereas in an insulator all of the electrons are tightly bound to their respective atoms. In an uncharged body, there are an equal number of positive and negative charges. In metals a few of the electrons are free to move from atom to atom, so that when a negatively charged rod is brought to the end of a conductor; it repels nearby free electrons in the conductor and causes them to move. They in turn repel free electrons in front of them and give rise to

a flow electrons all along the conductor. Hence it is not necessarily the electrons from the charged rubber rod that reach the electroscope leaf, but rather the electrons from the end of the wire where it touches the electroscope knob.

5. There are a large number of substances that are neither good conductor of electricity nor good insulators. These substances are called semiconductors. In them, electrons can move only with some difficulty, i.e., with considerable force.