Theory  of  Phasemeters. 
91 
one  good  hot-wire  ammeter  of  the  range  required  was 
available,  and  this  was  put  in  the  A1  circuit.  At  first  the 
wattmeter  method  of  testing  (f>  was  used,  a  Mather-Duddell 
instrument  being  utilized,  with  its  current-coil  placed  in  the 
Ax  circuit.  The  pressure  circuit  of  this  instrument  was  put 
in  parallel  with  the  moving-coil  circuit  of  the  phasemeter, 
and  with  a  Weston  dynamometer  voltmeter.  The  current 
circuits  were  not  altered  during  the  tests.  The  six  terminals 
of  the  alternator,  together  with  the  neutral  point,  formed 
seven  points,  any  two  of  which  could  be  selected  for  appli- 
cation to  the  pressure  circuits.  In  this  way  many  different 
values  of  <f>  could  be  obtained,  and  a  new  set  of  tests  could 
be  made  with  a  condenser  substituted  for  the  non-inductive 
resistance  in  series  with  the  moving  coil  of  the  phasemeter. 
Of  course,  when  interpreting  the  results  in  the  latter  case, 
the  value  of  cf>  as  deduced  from  the  wattmeter  readings  had 
to  be  changed  by  90  degrees  to  get  the  corresponding  phase- 
difference  between  the  moving  coil  current  and  the  current 
Ax.  The  wattmeter  tests  were  made  on  the  instrument 
shown  in  fig.  3,  but  not  with  the  particular  arrangement  of 
coils  there  indicated ;  and  the  results  found  were  in  close 
agreement  with  the  curve  between  x  and  <£  as  determined 
by  the  direct-current  method.  The  wattmeter  method  proved 
laborious  when  a  large  number  of  tests  were  needed ;  and  it 
was  found  sufficiently  accurate,  and  much  quicker  and 
simpler,  to  assume  that  the  six  terminals  of  the  alternator 
gave  voltages  in  the  precise  phase  relation  indicated  by  the 
geometrical  properties  of  fig.  4,  in  which  the  points  Al5  A2, 
A3  are  120°  apart  and  correspond  with  the  current  circuits, 
while  the  points  k^  and  QR  are  the  extremities  of 
two  perpendicular  diameters.  The  centre  of  the  circle,  N, 
denotes  the  neutral  point.     The  current   vectors  are  NAls 
