the  primary  bobbin  it  has  a  diminished  effect  in  disturbing  the
balance ;  in  the  case  of  iron  the  disturbance  is  increased  by
turning  the  disc  edgeways.  In  order  to  have  before  us  a
typically  simple  case,  imagine  an  induction  balance  made  of
two  very  long  primary  helices  and  each  embraced  near  the
centre  by  a  small  secondary  coil.  Let  the  primary  coils  be
traversed  by  a  simple  periodic  current.  We  have  then  in  the
interior  of  the  primary  coil  a  uniform  magnetic  field  varying
synchronously  with  the  primary  current  in  a  simple  periodic
manner,  and  the  rate  of  change  of  the  magnetic  field  at  any
instant  will  be  a  measure  of  the  electromotive  force  acting  in
the  secondary  circuit.  Suppose  into  one  primary  helix  is
inserted  a  thin  copper  tube ;  this  will  form  a  closed  secondary
circuit,  and  secondary  periodic  currents  will  be  induced  in  it,
flowing  round  the  cylinder  in  directions  parallel  to  the  turns
of  the  primary  helix.  As  this  copper  cylinder  possesses  a  very
sensible  time  constant^  the  phase  of  these  secondary  currents
in  the  copper  cylinder  will  be  nearly  opposite  to  that  of  the
primary  current.  The  resultant  magnetic  field  in  the  interior
of  the  cylinder  is  therefore  that  due  to  the  resultant  of  these
two  simple  periodic  currents  which  are  nearly  opposed  in
phase.  Hence  the  absolute  magnitude  of  the  interior  field
and  its  rate  of  variation  will  be  less  than  if  the  copper
cylinder  was  removed.  It  results,  therefore,  that  the  induction
through  the  secondary  helix  and  the  electromotive  force
impressed  on  it  will  be  diminished  by  the  presence  in  the
primary  coil  of  this  copper  cylinder.  The  diagram  given  on
page  177,  showing  a  geometrical  construction  for  the
magnitudes  of  the  primary  and  secondary  currents  in  an
induction  coil  without  iron,  shows  us  why  the  primary  and
secondary  currents  are  thus  more  or  less  opposite  in  phase.
