In  the  preceding  experiment  the  secondary  conductor  was
always  placed  between  the  wall  and  the  primary  conductor —
that  is  to  say,  in  a  space  in  which  the  direct  and  reflected
rays  were  travelling  in  opposite  directions,  and  gave  rise  to
stationary  waves  by  their  interference.
He  next  placed  the  primary  conductor  between  the  wall  and
the  secondary,  so  that  the  latter  was  in  a  space  in  which  the
direct  and  reflected  waves  were  travelling  in  the  same  direction.
This  would  necessarily  give  rise  to  a  resultant  wave,  the  inten-
sity of  which  would  depend  on  the  difference  in  phase  of  the
two  interfering  waves.  In  order  to  obtain  distinct  results  it  was
necessary  that  tue  two  waves  should  be  of  approximately  equal


DYNAMICAL  THEOBY  OF  INDUCTION.         477

intensities,  and  therefore  the  distance  of  the  primary  from  the
wall  had  to  he  small  in  comparison  with  the  extent  of  the
latter,  and  also  in  comparison  with  its  distance  from  the
secondary.
To  fulfil  these  conditions  the  secondary  was  placed  at  a  dis-
tance of  14  metres  from  the  reflecting  wall,  and,  therefore,
ahout  1  metre  from  the  opposite  one,  with  its  plane  in  the  plane
of  vibration,  and  its  air  space  directed  towards  the  nearest  wall,
in  order  to  make  the  conditions  as  favomrable  as  possible  for
the  production  of  sparks.  The  primary  was  placed  parallel  to
its  former  position,  and  at  a  perpendicular  distance  of  about
80  centimetres  from  the  centre  of  the  reflecting  metallic  plate.
