17. The Production of Electricity by Rotating Machinery.- Electricity can be produced dynamically, i.e., from motion and this method is always used when a large amount of electrical power is desired. It is produced in this manner by means of rotating machines called "generators" or "dynamos." In order to understand how electricity is produced by such a machine it is necessary to know something about the relation between electricity and magnetism.

In the discussion of magnetism it was stated that a magnetic field surrounded a magnetic pole, and that this field consisted of imaginary lines of force which extended from one pole to another. If a loop of wire is placed in the field in such a manner that some of the lines of force pass through the loop, nothing occurs as long as the loop is held stationary and the magnetic field kept at a constant strength, but, if the number of magnetic lines of force which pass through the loop of wire is changed, a voltage will be generated in the loop in proportion to the change in the number of lines through the loop in a second of time. The number of lines of force which pass through the loop can be changed by changing the position of the loop with respect to the magnet or by varying the strength of the magnetic field, but, if the position of the loop is changed without changing the number of lines of force through it, no voltage will be generated.

If the loop is closed, current will flow in the loop whenever a voltage is generated in it. Thus we have the prerequisites for electric power, namely, current and voltage. If a loop of wire is rotated in a magnetic field in such a manner that the number of lines of force through the loop is continuously changing in number, an alternating current is generated. In order to change the alternating to direct current a mechanical device known as a "commutator" is used. The production of electric power by generators will be discussed in more detail in Chapter III.

18. Alternating and Direct Currents.- In speaking as we have above, of "alternating" and "direct" current, the problem immediately presents itself -just what is meant by these terms "alternating" and "direct" as applied to electricity? What is termed a "pure direct current" is a very steady, even flow of current in one direction, such as is obtainable from a battery.

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