Y


322  TELEPHONY  WITHOUT  WIRES

To  secure  the  best  operation  of  these  detectors  in  the  manner
outlined  above,  it  is  necessary  that  the  grid  should  be  capable
of  absorbing  as  many  as  possible  of  the  electrons  passing
towards  it  when  it  is  positively  charged,  in  order  to  bring
about  a  decrease  in  the  plate  circuit  current.  It  must  not
merely  attract  the  electrons  without  absorbing  them,  as  that
would  bring  about  an  'increase  in  the  plate  circuit  current.1
For  this  purpose  the  grid  must  be  constructed  with  sufficient
surface  area,  that  is  to  say,  of  fairly  thick  wire,  while  in  some
cases  perforated  metal  sheet  has  been  adopted.  The  conditions
are  therefore  different  from  those  obtaining  in  the  similarly
constructed  amplifiers  2  in  which  it  is  desired  that  the  grid
shall  absorb  as  few  as  possible  of  the  electrons  passing  through
it.  For  the  amplifiers,  then,  very  fine  grid  wires  must  be
used.3

It  is  sometimes  found  useful  to  insert  a  small  condenser  C2
(Fig.  229)  in  series  with  the  grid  circuit  of  the  audion,  but  if
this  is  done  it  must  be  shunted  by  a  high -resistance  leak,  R2,
across  its  terminals  (unless  its  dielectric  should  be  sufficiently
leaky  without  this  addition),  in  order  to  form  a  path  for  the
electrons  that  are  absorbed  by  the  grid.  If  no  bye-path  of
this  nature  is  provided,  the  grid  becomes  strongly  negatively
charged  by  the  electrons  settling  upon  it,  and  as  a  result  the
entire  flow  of  current  to  the  plate  is  stopped  until  the  grid  is
discharged.  The  rate  of  absorption  and  removal  of  electrons
by  the  grid  can  be  controlled  to  a  certain  extent  by  varying
this  leak  resistance.  In  some  instances  it  is  possible  to  operate
an  audion  bulb  without  a  leak  of  this  nature  across  the  condenser
C2,  but  this  is  only  the  case  when  there  is  strong  ionisation  of
the  residual  gas  in  the  bulb,  so  that  there  are  a  considerable
number  of  positive  ions  in  the  neighbourhood  of  the  grid  to
neutralise  any  accumulation  of  negative  charge  upon  it.
