CHAPTER VII ELECTRICAL APPURTENANCES
Wires
Electric currents are usually led from place to place, at will, by means of conductors called wires. There are a great many kinds of wires, each adapted to some special purpose.
Wires are usually covered with a material called an insulator, in order to prevent the loss of electric current due to the wires coming into contact with other bodies or circuits. Insulators are substances which do not conduct electricity.
Wires which are insulated by heavy braids of cotton fiber and then impregnated with some compound, such as creosote, are called weather-proof wires, and are best adapted to outside service, where they must be exposed to the action of the elements.
The wires used for interior wiring in buildings, etc., are usually insulated with rubber, over which is placed a cotton braid to protect the rubber.
Rubber cannot well be used as an insulator for all wires, although its insulating value is very great, owing to the fact that it deteriorates under many conditions.
Rubber-covered and weather-proof wires are made in a variety of insulations. Some may have only one insulating layer, while others have a great many. Different substances are used as insulators to adapt the wire to some special purpose. Copper is usually the only metal used to form the wire or conductor itself. The reason for this is that copper is a better conductor than any other metal except those known as precious metals, such as gold and silver, the cost of which prohibits their use for such purposes. The wire may be solid, or made up of a number of small conductors so that it is flexible.
The various combinations of insulating layers, together with either a solid or a stranded conductor, have made possible a variety of current-carriers, known as: