It has been proved that electrons (negative charges) move through a conductor in response to an electric field. "Electric current" is defined as the directed flow of electrons and the direction of electron movement is from a region of negative potential to a region of positive potential. Therefore, electric current can be said to flow from negative to positive.

CONDUCTORS AND INSULATOR
Any material that will allow an electrical current to flow through it is an electrical conductor. Any material that blocks electrical current flow is an electrical insulator. Conductors are used in automotive equipment to carry electric current to all of the electrical equipment. Insulators also are necessary to keep the electric current from taking a shorter route instead of going to the intended component. The electrical properties of a substance depend mainly on the number of electrons in the outermost shell of each atom. The maximum number of electrons in an outer shell is eight. When there are less than four electrons in the outer shell of an atom, these electrons will tend to be free. This condition allows the free motion of electrons, making the substance a conductor (fig. 1-3).

Electrical energy is transferred through con-ductors by means of the movement of free electrons that migrate from atom to atom within the conductor. Each electron moves a short distance to the neighboring atom where it replaces one or more electrons by forcing them out of their orbits. The replaced electrons repeat this process in nearby atoms until the movement is transmitted throughout the entire length of the conductor, thus creating a current flow. Copper is an example of a good conductor because it only has one free electron. This electron is not held very strongly in its orbit and can break away from the nucleus very easily. Silver is a better conductor of electricity but it is too expensive to be used in any great quantity. Because of this, copper is the conductor used most widely in automotive applications.

Whenever there are more than four electrons in the outer orbits of the atoms of a substance, these electrons will tend to be bound, causing restriction of free electron movement, making it an insulator (fig. 1-3). Common insulating substances in automotive applications are rubber, plastic, and fiberboard.

Figure 1-3.- Conductors and insulators.