In  the  first  method,  the  current  of
radio-  or  audio-frequency  is  repeated
back  to  the  grid  circuit  by  special
radio-  or  audio-frequency  transform-
ers, but  in  the  second  method,  a  trans-
former is  connected  in  the  local  battery  circuit  and  its  secondary  terminals  con-
nected to  the  grid  and  filament  of  a  second  valve  by  which  the  incoming  radio  signal
is  amplified  by  repetition  of  the  action  shown  in  Fig.  180.
Fig.     181 — "Repeater"     or     "Regenerative"     Vacuum
Valve  Circuit.
RECEIVING  CIRCUITS,  DETECTORS,  TUNING  APPARATUS.
161


The  three  element  vacuum  valve  connected  in  a  simple  tuning  circuit  for  the  reception
of  spark  signals  is  adjusted  for  the  maximum  strength  of  signals  as  follows:  (1)  The
incandescence  of  the  lamp  filament  is  carefully  adjusted  by  the  filament  rheostat;  (2)  The
voltage  of  the  local  battery  is  varied  until  the  maximum  strength  of  signals  is  obtained.
To  protect  the  filament  and  assist  adjustment,  modern  sets  have  a  small  ammeter  connected
in  series  with  the  lamp  filament  by  which  the  degree  of  incandescence  is  definitely  fixed.
143.  A  Repeater  Vacuum  Valve  Circuit.— A  method  for  repeating  the
current  of  radio  frequency  flowing  in  the  telephone  circuit  back  to  the  grid  circuit
of  the  same  bulb  appears  in  Fig.  181.  This  system  has  been  found  to  be  particularly
successful  in  operation  and  gives  increased  strength  of  signals  from  a^given  sta-
tion. Part  of  the  energy  of  the  oscillations  of  radio-frequency  flowing  in  the
telephone  circuit  is  repeated  back  to  the  secondary  circuit  through  the  coupling
coils  (of  radio-frequency)  L-l  and  L-2,  and  the  amplitude  of  the  grid  oscillations
thereby  increased.  The  amplified  grid  oscillations  then  react  upon  the  telephone
circuit,  producing  still  greater  variation  of  the  local  current,  thus  further  reinforc-
ing the  oscillations  of  the  system.
