If  a  angle  coin  or  mass  of  metal  be  introduced
between  either  A  and  C,  or  B  and  D,  or  even
above  one  of  the  coils,  as  at  d,  the  balance
will  be  disturbed,  since  some  of  the  induction  is
now  expended  in  producing  electric  currents  in
the  interposed  metal,  and  a  sound  will  therefore
be  heard  in  the  telephone.  But  if  precisely  similar
metals  are  placed  in  similar  positions,  between  A
and  C,  and  B  and  D,  no  sound  is  heard  in  the
telephone,  since  the  inductive  effects  due  to  the
two  metals  are  the  same.
The  slightest  difference,  however,  either  in
composition,  size  or  position,  destroys  the  balance,
and  causes  a  sound  to  be  heard  in  the  telephone.
A  spurious  coin  is  thus  readily  detected  when
compared  with  a  genuine  coin.
A  somewhat  similar  instrument  has  been  em-
ployed to  detect  and  locate  a  bullet  or  other  for-
eign metallic  substance  in  the  human  body.
In  order  to  determine  the  amount  of  the  dis-
turbance, an  instrument  called  a  sonometer  is
used  (See  Sonometer,  Hughes1),  in  which  a  single
secondary  coil,  placed  in  the  circuit  of  a  telephone,
slides  on  a  graduated  bar  between  two  fixed
primary  coils,  so  wound  as  to  exert  equal  and  op-
posite inductions  on  the  secondary.  When,  there-
fore,  the  secondary  is  exactly  in  ihe  middle  of  the


Bal.]


40


[Bal.
