236 



ELECTRICITY ON A LARGE SCALE 



The dynamo. Between the poles of a strong magnet suspend 

 a movable coil which is connected with a sensitive galvanometer 

 (Fig. 107). Starting with the coil in the position of Figure 101, 

 when many lines of force pass through it, let the coil be rotated 

 quickly until it reaches the position indicated in Figure 107, 

 when no lines of force pass through it. During the motion of 

 the coil, a strong deflection of the galvanometer is 

 observed ; but the deflection ceases as soon as the 

 coil ceases to rotate. If, now, starting with the 

 position of Figure 107, the coil is rotated forward 

 to its starting point, a deflection occurs in the 

 opposite direction, showing that a current is 

 present, but that it flows in the opposite direc- 

 tion. As long as the coil is in motion, current 

 is induced in the coil. 



The above arrangement is a dynamo in miniature. 

 By rotation of a coil (armature) within a mag- 

 netic field, that is, between the poles of a magnet, 

 current is obtained. 



In the motor, current produces motion. In 

 the dynamo, motion produces current. 

 Every dynamo, no matter how complex its structure and 

 appearance, consists of a coil of wire which can rotate con- 

 tinuously between the poles of a strong magnet. 



The current obtained from such a dynamo alternates in direc- 

 tion, flowing first in one direction and then in the opposite direc- 

 tion. Such alternating currents are unsatisfactory for many pur- 

 poses, and to be of service are in many cases transformed into 

 direct currents; that is, currents which flow steadily in one 

 direction. This is accomplished by the use of a commutator. 



A small dynamo, such as is used for lighting fifty incan- 

 descent lamps, has a horse power of about 3 to 4, and large 

 dynamos are frequently as powerful as 7500 horse power. 



FIG. 107. As 

 long as the 

 coil rotates 

 between the 

 poles of the 

 magnet, cur- 

 rent flows. 



