297-  Magnetic  Fields  about  Parallel  Currents  (Th.,
385).  —  The  reciprocal  action  between  conductors  carry-
ing currents  is  purely  magnetic,  and  may  be  accounted  for
by  the  stresses  set  up  in  the  surrounding  medium.  The
magnetic  field  about  a  single  conductor  is  composed  of
concentric  circles  (284) ;  but  when  the  fields  of  two  con-
ductors are  in  part  superposed,  the  composite  magnetic
figures  will  be  those  due  to  the  resultant  of  the  two  sets
of  forces  in  every  part  of  the  field.  Moreover,  these  figures
will  exhibit  attraction  or  repulsion  between  the  conductors
according  to  the  relative  directions  of  the  currents  through
them.
344


ELECTRICITY    AND    MAGNETISM.
Fig.  159  is  the  field  developed  by  iron  filings  about  two
parallel  wires  passing  through  the  two  holes  and  with  the
currents  flowing  in  the  same  direction.  In  addition  to
the  distortion  of  the  small  circles  immediately  about  the
conductors,  showing  that  they  are  crowded  together  on
the  outward  sides  and  elongated  between  the  wires,  there
are  continuous  curves  enclosing  both  circuits.  These  are
due  to  the  coalescence  of  a  number  of  circles  belonging
to  the  two  currents.  The  conductors  are  drawn  together
by  the  tension  along  these  lines  of  force.  Midway  between
the  two  is  a  region  where  the  magnetic  forces  represented
by  the  circles  are  in  opposite  directions,  and  here  the  field
is  neutral.
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Fig.  159.
Fig.  160.
