In electrical parlance, that part of the current from _a_ to _c_ or from _c_ to _e_ is known as an _alternation_. From _a_ to _e_ is called a cycle. The reason why alternating current is often used in place of direct current is that it can be sent over the wires for long distances more economically than direct current. This is more fully explained farther on in the chapter dealing with a step-down transformer. The number of _cycles_ taking place in one second is known as the _frequency_ of the current. The usual _frequency_ of commercial alternating currents is 60 cycles per second or 7200 alternations per minute. 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: Theater or Stage Cable Elevator Cable Fixture Wire Telephone Wire Mining Cable Feeder Cable Brewery Cord Heater Cord, etc. depending upon the special use for which they were designed. The wires which the young experimenter is likely to use in his work the most are known as _magnet wires_, and are used for making electro-magnets, coils, and various windings. Magnet wires may be insulated with either silk, cotton, or enamel. Silk-covered and cotton-covered wires may be obtained with either a single or double covering.