Where the current has to be transmitted a long distance and the load is approximately constant, the alternating system is preferred, as it can be operated with small main lines or conductors. This effects a saving in copper, over the direct system which requires larger conductors. [Illustration: FIG. 219.] [Illustration: FIG. 220.] Fig. 217, on page 247, shows _a four-pole generator_ designed to run by a belt or directly connected to an engine. The five parts named, as the principal parts of a dynamo, are all shown in the figure. The machine is arranged ready to be bolted to the floor. Fig. 219 on the opposite page is the _armature_ which is made up of coils of insulated wire, the free ends of which, see Fig. 220, are united to the arms of the _commutator bars_. When the armature is finished, as shown, the wire forms an unbroken circuit. [Illustration: FIG. 221.] Fig. 221 is intended to represent another form of armature, but the principle upon which it operates, is the same, as the other shown. A A, represents the wire coils of the armature, B, is the shaft with its journals, C, is the commutator. All commutators both for generators and for motor armatures are insulated by mica between the bars. [Illustration: FIG. 222.] Fig. 222 shows _a woven wire brush_. The brushes on the dynamo, page 247, are made of carbon. [Illustration: FIG. 223.] Fig. 223 shows an alternating _induction_ motor. Induction is a property by virtue of which an electric current is transferred from one conducting line to another _without any metallic connection_; it is that influence by which a _strong current_ flowing through a conductor controls or affects _a weaker current_ flowing through another conductor in its immediate neighborhood,—the strong current remaining unaffected. [Illustration: FIG. 224.] Fig. 224 exhibits the armature for the above alternator; it is familiarly called a “squirrel cage armature” on account of its resemblance to the wheel in a squirrel’s cage. Its peculiar construction enables it to run without producing any sparks; this feature renders it safe to run where there are explosive gases which might be ignited by an electric spark. In the machine the bearings are cast solid with the end shields, thus assuring perfect alignment when properly turned. Another feature is the automatic self adjusting bearings which are lubricated mechanically by rings resting upon the shaft. These rings were formerly a failure, but by the use of mineral oils are now a success. This machine is one of the simplest designs of alternating motors, the example, Fig. 223, is one developing one hundred horse-power. [Illustration: FIG. 225.] Fig. 225 shows a revolving _field_ with “spider.” In this construction of generators or motors the field revolves in place of the armatures, the first object of this design is to reduce the high rotative speed; it is also claimed to have a better electrical efficiency.