for  Rectifying  High-Frequency  Electric  Currents.        663 
fields.  For  if  we  interpose  between  the  primary  and 
secondary  circuits  a  sheet  of  tinfoil  or  zinc,  we  see  a  notable 
decrease  in  the  galvanometer  deflexion,  thus  making  the 
screening  action  of  the  metal  very  evident.  Employed  in 
this  manner,  it  enables  us  to  show  strikingly  the  rapid  rate 
at  which  the  field  due  to  a  current  in  a  square  or  circular 
circuit  decreases  with  distance  from  the  circuit,  and  therefore 
to  illustrate  one  of  the  disadvantages  under  which  wireless 
telegraphy  by  electromagnetic  induction  labours  when  com- 
pared with  space  telegraphy  by  electric  waves.  When  using 
the  valve  to  detect  the  oscillations  in  an  antenna  produced 
by  the  impact  of  Hertzian  waves,  an  oscillation  transformer 
is  inserted  in  the  circuit  of  the  receiving  antenna,  and  its 
secondary  circuit  is  connected  through  a  valve  with  a  dead- 
beat  mirror-galvanometer.  We  are  thus  able  to  receive 
signals  over  short  distances  by  the  direct  effect  of  the  rectified 
oscillations  themselves  on  the  galvanometer. 
The  action  of  other  substances  besides  incandescent  carbon 
as  a  cathode  in  a  vacuum- value  has  also  been  studied.  It 
has  been  found  by  G.  Owen*  and  by  A.  Wehneltf  that 
glowing  metallic  oxides,  including  the  rare  oxides  employed 
in  the  manufacture  of  the  Nernst  lamp-glowers,  copiously 
emit  negative  ions  when  incandescent  both  at  atmospheric 
and  at  reduced  pressures.  Wehnelt  found  that  the  incan- 
descent oxides  of  calcium,  barium,  and  strontium  produce  an 
abnormally  powerful  electronic  discharge,  and,  following  the 
recommendations  of  the  author,  he  has  proposed  to  employ 
vacuum-tubes  with  one  electrode  covered  with  such  oxides 
and  heated,  as  rectifying  valves  for  alternating  currents. 
As  far  back  as  1890  the  writer  showed  in  a  lecture  ex- 
periment at  the  Eoyal  Institution  that  the  so-called  Edison 
effect,  that  is  the  passage  of  negative  electricity  across  space 
from  an  incandescent  carbon  filament  to  a  metallic  plate  near 
it,  could  take  place  at  atmospheric  pressure  if  the  plate  was 
very  near  the  filament.  It  is  easy  to  show  a  similar  experi- 
ment with  a  Nernst  electric  glow-lamp.  If  a  Nernst  lamp 
is  supported  with  the  bare  glower  horizontal  and  placed 
within  a  few  millimetres  of  a  vertical  insulated  metal  tube 
kept  cold  by  being  filled  with  water,  it  is  found  that  negative 
electricity  will  pass  freely  across  the  glower  to  the  cold 
metal,  but  not  in  the  opposite  direction.  Hence  if  the 
glower  and  metal  tube  are  inserted  as  a  gap  in  an  electric 
circuit  containing  a  sensitive  galvanometer,  and  if  secondary 
*  See  G.  Owen,  Phil.  Mag.  vol.  viii.  p.  230  (1904).  "On  the  Dis- 
charge of  Electricity  from  a  Nernst  Filament." 
t  See  A.  Wehnelt,  Phil.  Mag.  vol.  x.  p.  80  (1905).  «  On  the  Discharge 
of  Negative  Tons  by  Glowing  Metallic  Oxides  and  Allied  Phenomena." 
