Electricity is a form of energy, a phenomenon that is a result of the existence of electrical charge.  The theory of electricity and its inseparable effect, magnetism, is probably the most accurate and complete of all scientific theories.  The understanding of electricity has led to the invention of motors, generators, telephones, radio and television, X-ray devices, computers, and nuclear energy systems.  Electricity is a necessity to modern civilization. 

Electric Charge 

Amber is a yellowish, translucent mineral.  As early as 600 BC the Greeks were aware of its peculiar property:  when rubbed with a piece of fur, amber develops the ability to attract small pieces of material such as feathers.  For centuries this strange, inexplicable property was thought to be unique to amber. 

Two thousand years later, in the 16th century, William Gilbert proved that many other substances are electric (from the Greek word for amber, electron) and that they have two electrical effects.  When rubbed with fur, amber acquires resinous electricity;  glass, however, when rubbed with silk, acquires vitreous electricity.  Electricity repels the same kind and attracts the opposite kind of electricity.  Scientists thought that the friction actually created the electricity (their word for charge).  They did not realize that an equal amount of opposite electricity remained on the fur or silk. 

In 1747, Benjamin Franklin in America and William Watson (1715-87) in England independently reached the same conclusion: all materials possess a single kind of electrical "fluid" that can penetrate matter freely but that can be neither created nor destroyed.  The action of rubbing merely transfers the fluid from one body to another, electrifying both.  Franklin and Watson originated the principle of conservation of charge:  the total quantity of electricity in an insulated system is constant. 

Franklin defined the fluid, which corresponded to vitreous electricity, as positive and the lack of fluid as negative. Therefore, according to Franklin, the direction of flow was from positive to negative--the opposite of what is now known to be true.  A subsequent two-fluid theory was developed, according to which samples of the same type attract, whereas those of opposite types repel.