2. _The field magnets_, which produce the magnetic field in which the armature turns. 3. _The pole-pieces._ 4. _The commutator or collector._ 5. _The collecting-brushes_ that rest on the commutator cylinder and take off the current of electricity generated by the machine. Fig. 218 shows a dynamo of the early Edison type—the names of the principal parts are given in the note below, as well as those of the other parts of the machine. [Illustration: FIG. 218.] _This is a two-pole machine_, direct current; the figure is introduced to show the “parts” only—as this dynamo has been largely superseded by others of the four pole type. NOTE.—A, Magnet yoke; B, Magnet and field piece; C, Pole piece; D, Zinc field piece; E, Armature; F, Commutator; I, Quadrant; JJ, Brushes; K, Adjusting handle for the brushes; L, Switch pivot; M, Pilot lamp receptacle; N, Negative lug; O, Switch lever; P, Positive lug; Q, Positive terminal; R, Negative terminal; S, Negative rod; T, Pole piece; UU, Bearings; X, Slides for belt tightener; VVV, Driving pulley; Y, Connecting blocks, one on each side of machine. _An electric motor_ is a machine for converting electrical energy into mechanical energy; in other words it produces mechanical power when supplied with an electric current; a certain amount of energy must be expended in driving it; the _intake_ of the machine is the term used in defining the energy expended in driving it; the amount of power it delivers to the machinery is denominated _its out-put_. The difference between the out-put to the intake is the real _efficiency_ of the machine; it is well known that the total efficiency of _an electric distribution system_, which may include several machines, usually ranges from 75 to 80 per cent., at full load, and should not under ordinary circumstances fall off more than say 5 per cent. at one-third to half load; the efficiency of motors varies with their size, while a one horse-power motor will, perhaps, have an efficiency of 60 per cent., a 100 horse-power may easily have an efficiency of 90 per cent. and the larger sizes even more. 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—