VII.  Effect  of  Slots  on  Magnetic  Flux 

53.  With  slotted  armatures  the  pole  face  density  opposite  the 
armature  slots  is  less  than  that  opposite  the  armature  teeth,  due 
to  the  greater  distance  of  the  air  path  in  the  former  case.  Thus, 
with  the  passage  of  the  armature  slots  across  the  field  pole  a 
local  pulsation  of  the  magnetic  flux  in  the  pole  face  is  produced, 
which,  while  harmless  with  laminated  field  pole  faces,  generates 
eddy  currents  in  solid  pole  pieces.  The  frequency  of  this  pul- 
sation is  extremely  high,  and  thus  the  energy  loss  due  to  eddy 
currents  in  the 'pole  faces  may  be  considerable,  even  with  pul- 
sations of  small  amplitude.  If  S  =  peripheral  speed  of  the  arma-