INTRODUCTION

Historical Background on Discovery of Electricity

Perhaps the first "electrical" experiment conducted by man was performed by the Greek Philosopher Thales (640–546 B.C.). Thales observed that when a piece of a natural resin called amber was rubbed with a cloth, the amber attracted light objects near it. Today we say that rubbing amber with cloth gives the amber a charge of static electricity. The term "electricity" comes from "electra," the Greek word for amber.

Other substances also can be charged with static electricity by rubbing, such as a glass rod with silk, or hard rubber with fur. A familiar experience due to the creation of static electricity is the shock sometimes received in touching a doorknob or some other grounded object after walking across a carpet on a dry winter day.Static electricity, built up by tires rolling over pavement, dictates the use of a dangling chain or strap attached to trucks carrying flammable vapors to prevent sparks between the truck and the ground, which are a possible fire hazard.

Static electricity, as such, is not of primary interest here. However, the basic cause of its formation does have an important bearing on understanding the nature of electricity in forms, which are of practical interest.

To understand what happens, we must consider the structure of atoms, the smallest particles of matter. The atom, Figure 1, is made up of a positively charged

whirl very small, negatively charged particles called electrons. The number of negatively charged electrons is the same as the number of positive charges on the nucleus, so that the atom is stable, that is, it is not charged electrically. However, if for some reason the number of electrons is greater than that of the positive charges on the nucleus, the atom is negatively charged. On the other hand, if the number of positive charges in the atom is greater than the number of electrons the atom is said to be positively charged. When a hard rubber rod is rubbed with fur, electrons are stripped from the fur atoms. As a result, the rubber gains electrons and becomes negatively charged. The fur loses electrons, so becomes positively charged.

Charged atoms naturally tend to return to the stable, uncharged state. If one of two bodies has more free electrons than the other, the first body is said to be negatively charged with respect to the second, and the second is said to be positively charged with respect to the first. If the two bodies are connected together by a wire, electrons flow from the electrically negative body to the positive one. (We speak of this flow of electrons as electricity.)

Since the discovery of static electricity over 2500 years ago by Thales, many men have made other important electrical discoveries. Two of these discoveries have had a particularly important bearing on man's use of electricity.