The general and growing application of electric power to the driving of all kinds of machinery including pumps makes _the question of motor driving one of the most important in the power field_. For many purposes, a single speed is sufficient, but for others, it is imperative that the speed should be variable; and for still others, though not absolutely necessary, a speed adjustment is very desirable.
While the _direct-current motor_ has been in this field so long that its properties are well known and its possibilities fully developed, in the operation of motors located in the immediate neighborhood of the generator the _alternating-current motor_ has marked advantages where a large area of territory has to be covered and the conditions are nearly uniform, that is to say—
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.]
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.
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.
Fig. 222 shows _a woven wire brush_. The brushes on the dynamo, page 247, are made of carbon.
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.