THE DYNAMO 349 



ance, 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. 



The current obtained from such a dynamo alternates in 

 direction, flowing first in one direction and then in the oppo- 



FlG. 239. A modern electrical machine. 



site 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 commu- 

 tator (Section 310); in the construction of a dynamo, continu- 

 ous current in one direction is obtained by the use of a similar 

 device. 



