DIRECT-CURRENT. DYNAMOS..AND..MOTORS. 7 
effective cross-section or magnetic carrying capacity of 
such a core is from 15 to 5 per cent. less than that of an 
equal volume of solid iron of the same permeability. 
The effective cross-section of the armature core is further 
reduced by the presence of ventilating spaces, or holes, 
which are often left in the armature core. 
41. Advantages of Toothed Armatures.—In the 
toothed or slotted armature (Fig. 1), which is now 
largely superseding the smooth form, the conductors 
are wound in the slots between teeth or projections 
formed upon the main body of the core. This type, also 
Fic, 1.—TooTHED ARMATURE, 
called the Pacinotti armature, after its inventor, has 
the following advantages over the smooth core: 
_1. The reluctance of the air-gap is reduced to a minimum. 
2. The armature conductors are protected from injury. 
3. The conductors are firmly held in place and cannot slip 
on the core by the action of the electro-dynamic force ex- 
erted upon them. 
4, Eddy currents in the armature inductors are avoided, 
since the magnetic lines snap across the latter instantly.* 
5. If the teeth are practically saturated by the field magnet- 
ism, they oppose the shifting of the lines by armature 
reaction. . 
*This point will be considered further under the head of Eddy cur- 
PENS ei" _. rents in armature inductors. ° 
