Variations  of  e.m.f.  across  the  terminals  of  S
will  affect  A,  which  is  calibrated  in  amperes  by
comparison  with  a  standard  or  by  calculation.
The  Weston  Voltmeter.  The  principle  of  the
Weston  voltmeter  or  ammeter  for  direct  current
is  that  of  the  D'Arsonval  galvanometer.  The  mov-
ing coil  is  mounted  on  pivots  between  jeweled  cen-
tres instead  of  being  suspended  from  wires  or
threads.  This  mounting  permits  of  portability
and  compactness.
42


ELECTRICAL    INSTRUMENTS.
The  construction  of  the  moving  system
magnet  pole  pieces  is  shown  in  Fig.  29.
The  portable  type  of  Weston  voltmeter
ammeter  is  shown  in  Fig.  30.
and


and


Fig.  29.
In  the  portable  instruments  the  data  of  resist-
ance, etc.,  will  be  found  printed  on  the  inside  of
the  lid.     Care  should  be  taken  not  to  interchange


VOLTMETERS  AND  AMMETERS,


43


the  case  lids,  each  instrument  bears  a  serial  ntmi-
ber.
The  laboratory  type,  Fig.  31,  has  a  very  large
scale  of  peculiar  design,  Fig.  32,  which  admits  of
readings  being  made  to  a  fraction  of  one  degree  of
deflection.
Types  of  Weston  switchboard  instruments  are
shown  in  Figs.  33  and  34,  the  latter  being  made
to  fit  flush  with  the  face  of  a  switchboard.
Figs.  35  and  36  show  forms  which  have  trans-
parent dials  illuminated  from  behind.
Instruments  of  this  pattern  are  often  mounted
on  a  swinging  arm  that  they  may  be  read  from
various  parts  of  the  room.
A  black  disc  controlled  by  a  button  in  the  middle
of  the  case  front,  is  set  to  a  determined  point.
44


ELECTRICAL    INSTRUMENTS.
VOLTMETERS  AND  AMMETERS.
45


The  pointer  is  then  more  readily  located  as  to  its
proximity  to  this  disc  when  viewed  from  a  distance.
