not  only  on  the  magnitude  of  the  current,  but  also  on  its  phase
relation  to  the  emf.,  which  is  indicated  by  the  power  factor.  A
lagging  current  causes  a  greater  reduction  in  terminal  voltage  than
the  same  number  of  amperes  in  phase,  the  effect  increasing  with
the  lag.  Thus  at  80  per  cent  power  factor  it  may  be  twice  as  great
as  at  100  per  cent.  Conversely,  a  leading  current,  such  as  is  taken
by  condensers,  improves  the  regulation,  so  that  the  terminal  voltage
may  actually  be  higher  when  current  is  flowing  than  when  there  is
none.
84.  Effect  of  Speed  on  Regulation. — Since  the  emf.  is  proportional
to  the  rate  of  cutting  of  flux,  it  follows  that  fluctuations  of  speed
are  attended  with  proportional  fluctuations  of  voltage,  provided
the  field  excitation  is  not  changed  at  the  same  time.
85.  Voltage  Control. — The  simplest  way  to  control  the  voltage
of  a  generator  is  by  adjusting  the  strength  of  the  magnetic  field
by  means  of  the  field  current.     For  this  purpose  an  adjustable
resistance  is  inserted  in  the  circuit  of  the  latter,   called  a  field
rheostat.
One  kind  consists  of  a  quantity  of  wire  of  an  alloy  having  a
comparatively  high  resistance,  mounted  on  insulating  supports
in  a  perforated  iron  box  with  a  slate  face,  or  embedded  in  an  insu-
lating enamel.  A  handle  is  provided  for  making  contact  with  any
one  of  a  number  of  brass  studs  attached  to  the  resistance  wire  at
various  points,  so  that  more  or  less  of  it  can  be  in  circuit.  Ter-
minals are  provided  for  connecting  the  rheostat  to  the  field  circuit.
Fig.  109  shows  the  principle.
