ARMATURE WINDINGS. 419 



armature for a multipolar field magnet, the wire is wound length- 

 wise on the outside surface of the core and across the ends along 

 chords of nearly 90 for a 4-pole field magnet, along chords of 

 nearly 60 for a 6-pole field magnet, along chords of nearly 45 

 for an 8 -pole field magnet, and so on. The essential difference 

 between the drum armature and the ring armature is that in the 

 drum armature there are no return conductors in the interior of 

 the core ; but each conductor on the outside surface of the core 

 is connected to a return conductor which is also on the outside 

 surface and nearly 360 // from the first conductor, where p is 

 the number of field poles ; whereas, in the ring armature each 

 outside conductor is connected to a return conductor on the 

 inside of the ring. 



(3) Disk armatures. The characteristic feature of the disk armature is that the 

 conductors are arranged in a plane like the spokes of a wheel. These conductors 

 move between two or more pairs of opposite magnet poles which face each other on 

 opposite sides or faces of the disk-like armature. These radial wires are connected 

 at their inner ends in the same manner as are the ends of the conductors of a drum 

 armature at the commutator end of the armature, and the outer ends of the radial 

 wires are connected to each other in the same manner as are the ends of the con- 

 ductors of a drum armature at the end of the armature opposite to the commutator. 

 Disk anratures are seldom used nowadays in American practice. 



The drum winding is the type which is most used. The great 

 advantage of the drum winding is that, having no conductors 

 which pass through the interior of the armature core, it is possi- 

 ble to wind the armature coils on forming spools, tape, varnish, 

 and bake them, and slip them into place on the armature core as 

 shown in Figs. 48 and 49, Chapter II. 



40. Open and closed coil armatures. In most direct current 

 armatures the winding constitutes a closed circuit of wire as in 

 the simple ring winding shown in Fig. 17, Chapter II. Such 

 armatures are called closed coil armatures. In some direct cur- 

 rent armatures, for example, the armatures of the Brush, the 

 Thomson-Houston, and the Westinghouse arc light generators, 

 and in the armatures of most alternators, the windings do not con- 



