While the exact nature of electricity is unkown, a great deal is known about what it can do. By the mere closing of a switch, buildings are lighted, wheels are turned, ice is made, food is cooked, distant voices are heard, and countless other tasks—ordinary and extraordinary—are performed. Although a great number of uses for electricity have been discovered and applied, the field is by no means exhausted. Electric machines and devices that have been in use for many years are being improved and are now finding wider fields of application. Extensive research is constantly bringing forth and developing new devices. Much is still to be learned about electricity.
Electricity is a convenient form of energy. It is well known that when fuels such as coal, oil, and gas are burned, energy is released. A waterfall, whether it is man-made or natural, also possesses energy. Yet, to be of value, this energy must be made available at points where it can be used conveniently. Electricity furnishes the most practicable and convenient means yet devised for doing this. The energy of burning fuel or of falling water is changed to a more convenient form—electricity—by electric machines. It is transmitted to distant points over electric circuits. It is controlled by other electric machines. At points where it is to be used, it is converted into useful work by still other electric machines and devices.