ELECTRICAL MACHINERY 481 



reference to Fig. 357 of a ring armature it will be noticed 

 that the inside parts of each coil on the armature do not 

 cut lines of force, hence these lines conduct only the cur- 

 rent and may be known as so much dead wire. In the 

 drum-wound armature both sides of the coil cut lines 



FIG. 362 DRUM ARMATURE 



of force and the only dead wire is across each end. Al- 

 though the drum-wound armature has less dead wire 

 than the ring-wound, it is not as convenient to repair. 

 For this reason high-voltage direct-current arc-lighting 

 dynamos are generally constructed with ring armatures. 

 A combination of the two, which is known as a drum- 

 wound ring armature, is extensively used in practice. 



644. Self-exciting principle of dynamos. In the earlier 

 types of dynamos the field magnets were always sepa- 

 rately excited by either a battery or a magneto. Later it 

 was learned that the soft iron of the field magnet after 

 once being excited retains some of the magnetism. Since 

 then all direct-current dynamos are built on this principle. 

 There is sufficient magnetism remaining in the fields so 

 that when the armature is up to speed it cute enough 



