_The above arrangement is a dynamo in miniature_. By rotation of a coil (armature) within a magnetic field, that is, between the poles of a magnet, current is obtained. In the _motor_, current produces motion. In the _dynamo_, motion produces current. 321. The Dynamo. As has been said, the arrangement of the preceding Section is a dynamo in miniature. Every dynamo, no matter how complex its structure and appearance, consists of a coil of wire which can rotate continuously between the poles of a strong magnet. The mechanical devices to insure easy rotation are similar in all respects to those previously described for the motor. [Illustration: FIG. 239.--A modern electrical machine.] The current obtained from such a dynamo alternates in direction, flowing first in one direction and then in the opposite direction. Such alternating currents are unsatisfactory for many purposes, and to be of service are in many cases transformed into direct currents; that is, current which flows steadily in one direction. This is accomplished by the use of a commutator. In the construction of the motor, continuous _motion_ in one direction is obtained by the use of a commutator (Section 310); in the construction of a dynamo, continuous _current_ in one direction is obtained by the use of a similar device. 322. Powerful Dynamos. The power and efficiency of a dynamo are increased by employing the devices previously mentioned in connection with the motor. Electromagnets are used in place of simple magnets, and the armature, instead of being a simple coil, may be made up of many coils wound on soft iron. The speed with which the armature is rotated influences the strength of the induced current, and hence the armature is run at high speed. [Illustration: FIG. 240.--Thomas Edison, one of the foremost electrical inventors of the present day.] A small dynamo, such as is used for lighting fifty incandescent lamps, has a horse power of about 33.5, and large dynamos are frequently as powerful as 7500 horse power.