22.  Intrinsic  Magnetisation,  Etc.  Corresponding  to  intrinsic
electrisation,  intrinsic  electric  intensity,  etc.,  are  intrinsic  magnet-
isation, of  which  we  have  an  example  in  every  magnet,  intrinsic
magnetic  intensity  or  force  (denoted  by  Ji)  maintaining  the  induc-
tion, etc.  Intrinsic  magnetic  phenomena  do  not  appear  in  most
substances,  but  are  far  more  marked  than  the  corresponding  elec-
tric phenomena  in  others,  notably  iron,  nickel,  and  cobalt,  reach-
ing their  maximum  development  in  hard  steel.  The  magnetisation
of  these  substances  will  be  briefly  discussed  in  §§  35-39,  XIII.
Permanent  Magnets.  A  permanent  magnet  is  a  magnet  whose
pole  strengths  at  any  definite  temperature  are  constant  independ-
ently of  the  time,  of  the  nature  of  the  surrounding  medium,  and
of  the  proximity  of  other  magnets  and  electric  currents.  Such  a
magnet  is  purely  ideal,  but  approximately  permanent  magnets
can  be  made  of  long  cylinders  of  properly  tempered  hard  steel.
Such  a  magnet,  if  not  brought  into  too  strong  magnetic  fields,
and  if  kept  when  not  in  use  in  an  iron  case  will  retain  its  moment
constant  in  air,  even  when  the  ratio  of  its  length  to  the  square  root
of  its  cross -section  is  less  than  ten  to  one,  for  at  least  a  year  within
one  tenth  per  cent.  (Klemencic,  Ann.  der  Physik,  XII. ,  174,
1901).  From  the  discussion  of  the  permanent  electret,  §  8,  VI.,
it  is  clear  that  a  magnet  will  be  more  nearly  permanent  the
greater  the  ratio  of  its  length  to  its  diameter  (or  other  linear  di-
mensions perpendicular  to  its  length).  It  is  also  evident  that,
other  things  being  equal,  the  greater  the  coercive  force  (§25,
XIII.)  of  a  substance  or  the  "harder"  it  is  magnetically,  the
nearer  will  be  its  approach  to  permanency  of  magnetisation,
when  made  up  into  magnets.
280          ELEMENTS    OF    ELECTROMAGNETIC    THEORY.
