The  transmission  of  rapidly  varying  or  sim-
ple-periodic currents  through  conductors  differs
rery  greatly  from  the  transmission  of  steady  cur-


rents. With  a  steady  current,  the  current  density
is  the  same  for  all  areas  of  cross-section  of  the
conductor.  For  a  rapidly  intermittent  current,
the  current  density  is  greater  near  the  surface,
and  when  the  rate  of  intermission  is  sufficiently
great,  the  current  is  entirely  absent  at  the  centre
of  the  conductor.
Lord  Rayleigh  has  shown  that  when  the  rate  ot
intermission  is  1,050  per  second,  the  effective  re-
sistance of  a  wire  i6omm.  in  length,  and  30  mm.
in  diameter,  is  i .  84  times  its  resistance  to  steady
currents.  He  found  that  the  increase  of  resist-
ance is  greater  in  the  case  of  conductors  of  great
diameter  than  in  those  of  small  diameter.
As  regards  the  character  of  conductor  best
suited  for  transmitting  rapidly  alternating  cur-
rents, it  can  be  shown  :

(I.)  That  for  transmitting  alternate  currents  o<
moderate  frequency,  say  of  about  1,000  per  sec-
ond, copper  conductors  should  be  used  in  prefer-
ence to  rods  of  iron.
(2.)  That  the  conductor  should  be  in  the  form
of  thin  strips,  or  if  tubular,  of  thin  walls.
(3.)  That  the  mere  stranding  of  the  conductor,
z.  e.,  forming  it  of  separate  insulated  conductors
connected  in  parallel,  will  be  of  no  effect  in  pre  •
venting  the  current  from  acting  on  the  outside  of
the  conductor,  unless  the  conductor  be  arranged
in  the  form  of  a  cable,  in  which  one  part  forms  a
lead,  and  another  part  the  return.
