The  movable  coil  is  suspended  by  means  of  a
thread  and  a  delicate  spring,  S,  capable  of  being
twisted  by  turning  a  milled  screw  -head  through
an  angle  of  torsion  measured  on  a  scale  by  means
of  an  index  connected  to  the  screw-head.  The
two  ends  of  the  movable  coil  dip  into  mercury
cups  so  connected  that  the  current  to  be  measured
passes  through  the  fixed  and  movable  coils  in
series.
When  ready  for  use  the  movable  coil  is  at  right
angles  to  the  fixed  coil.  The  current  to  be  meas-
ured is  then  sent  into  the  coils,  and  their  mutual
action  tends  to  place  the  movable  coil  parallel  to
the  fixed  coil  against  the  torsion  of  the  spring,  S.
The  amount  of  this  force  can  be  ascertained  by
determining  the  amount  of  torsion  required  to
bring  the  movable  coil  back  to  its  zero  position.
Fig,  224..     Siemens'  Electro- Dynamometer,

current  to  be  measured.  The  deflecting  force,
and  consequently  the  current  strength,  is  there-
fore  proportional  to  the  square  root  of  the  angle
of  torsion,  and  not  directly  to  the  angle  of  tor-
sion.
Dyne. — The  unit  of  force.
The  force  which  in  one  second  can  impart
a  velocity  of  I  centimetre  per  second  to  a
mass  of  I  gramme.
The  dyne  is  the  unit  of  force,  or  a  force  capa-
ble, after  acting  for  one  second  on  a  mass  of  I
gramme,  of  giving  it  a  velocity  of  I  centimetre
per  second.  The  weight  of  a  body  in  dynes,  or  the
force  with  which  it  gravitates,  is  equal  tj  its
mass  in  grammes,  multiplied  by  the  acceleration
imparted  to  it  in  centimetres  per  second.  For
this  latitude  the  acceleration  is  about  981  centi-
metres per  second.
E,]


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