The word generator is derived from a word meaning birth giving, hence also the dynamo is the machine generating or giving birth to electricity.
Again, the dynamo is a machine driven by power, generally steam or water power, and _converting the mechanical energy expended in driving it, into electrical energy of the current form_.
To summarize, the dynamo-electric generator or the dynamo-electric machine, proper, consists of five principal parts, viz:
2. _The field magnets_, which produce the magnetic field in which the armature turns.
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.
_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a two-pole machine_, direct current; the figure is introduced to show the “parts” only—as this dynamo has been largely superseded by others of the four pole type.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.