are  (1)  the  density  of  the  magnetic  field  and  (2)  the  speed  of  the  dynamo  arma-
ture per  second.  We  see  from  this  that  the  voltage  of  any  generator  may  be
increased  by  increase  of  the  speed  of  the  armature  or  by  increase  of  the  strength
of  the  magnetic  field  surrounding  the  armature  coils.  Commercial  generators  are
built  usually  for  constant  speed.  Regulation  of  the  voltage  is  obtained  by  means
of  the  field  rheostat.
35.  Diagram  of  an  Alternating  Current  Dynamo. — The  essential  parts  of
an  alternating  current  dynamo  are:

(1)  Field  Magnets.
(2)  Armature.
(3)  Collector  Rings.
The  diagram  of  Fig.  23  is  merely  intended  to  show  the  general  details  of  the
construction  and  connections  of  an  alternating  current  dynamo.  The  field  poles
which  are  firmly  bolted  to  the  circular  iron  frame  are  represented  by  N,  S,  N,  S,
the  armature  at  M  and  the  collector  rings  at  E,  F.  The  field  poles  are  wound
alternately  in  opposite  directions  so  that  the  current  circulates  about  the  turns  in
opposite  directions,  giving  the  poles  alternately  north  and  south  polarity.  The
armature  M  is  built  up  of  a  number  of  slotted  sheets  of  soft  iron  which  are  pinned
together  and  mounted  on  a  common  shaft,  the  copper  conductors  lying  lengthwise
of  the  core  in  such  a  way  that  the  coils  will  be  filled  and  emptied  with  magnetic
flux  (coils  not  shown).  If  these  coils  are  properly  connected  together,  the
currents  induced  therein  (by  the  change  of  flux)  will  flow  in  the  same  gen-
eral direction,  the  voltage  of  one  coil  being  added  on  to  that  of  the  next  coil.
