If  they  are  carefully  made  and  precisely  measured  they  'are  often


called  standard  resistance  coils.  Such  standards  may  be  secured
in  range  from  0.00001  ohm  to  100,000  ohms,  and  of  any  desired  cur-
rent carrying  capacity  and  degree  of  precision.
Banks  of  incandescent  lamps  in  various  arrangements  are  often
used  as  resistors.  The  resistance  of  such  lamps  is  subject  to  large
variations  in  value  with  changes  in  temperature.  However,  when
operating  under  steady  conditions,  either  hot  or  cold,  they  are
satisfactory  for  many  purposes.  Such  a  rheostat  offers  the  advantage
of  being  readily  adjustable  by  turning  lamps  off  or  on.  It  is  com-
pact and  there  is  no  danger  of  overheating.
11.  Conducting  Materials. — Conducting  materials,  usually  metals
or  metallic  alloys,  are  utilized  in  electric  circuits  with  two  different
purposes  in  view.  In  one  case  a  high  degree  of  conductivity  is


RADIO  COMMUNICATION.  33

required,  while  in  the  other  case  relatively  high  resistivity  is  de-
sired. These  cases  will  be  discussed  in  turn.
(a)  If  the  conductor  is  transmitting  energy  to  a  distant  point  by
means  of  the  electric  current  it  is  seen  from  equation  (2)  that  some
energy  will  be  wasted  in  the  conductor  in  the  form  of  heat.  This
loss  should  be  kept  as  small  as  possible,  and  to  this  end  great  care
is  taken  in  choosing  the  size  and  material  of  the  conductor.  For
reasons  of  economy  the  cross  section  must  not  be  too  great,  hence  a
desirable  material  for  conducting  lines  must  have  low  resistivity
and  must  be  abundant  and  relatively  cheap  to  produce.  Such  a
material  is  copper.  Where  lightness  is  important  and  where  in-
creased dimensions  are  not  a  disadvantage,  aluminum  is  much  used.
