Oscillograph  records  taken  with  the  apparatus  in  operation


TYPES  OF  MICROPHONES


275


have  indicated  an  amplification  of  about  350  :  1  between  the
voltamperes  used  in  the  microphone  control  circuit,  and  the
actual  H.F.  energy  that  is  modulated.
By  proper  adjustment  of  the  values  of  the  shunt  and  series
condensers  used  with  the  instrument,  the  relation  between
the  direct  current  through  the  controlling  winding  and  the
energy  output  from  the  alternator  can  be  made  almost  a
linear  one.  This  is  a  valuable  feature  from  the  point  of  view
of  "  distortionless  "  telephony.
FIG.  197.— General  Scheme  of  "Shunt  Valve"  Microphonic  Control  of
Oscillation  Generator.
When  large  outputs  of  H.F.  energy  have  to  be  dealt  with,
it  is  desirable  to  amplify  the  initial  microphone  modulations
before  utilising  them  to  control  the  magnetic  microphone.
This  is  in  order  to  obtain  sufficient  variation  in  the  ampere -
turns  on  the  control  winding  to  effectively  control  the  high-
frequency  currents.  This  preliminary  amplification  may.  if
necessary,  be  effected  by  one  of  the  forms  of  audion  or  pliotron
amplifiers  (see  also  p.  302).
276  TELEPHONY  WITHOUT  WIRES

SHUNT  VALVE  MICROPHONIC  CONTROL

The  General  Electric  Company  (U.S.A.)  have  developed  a
mode  of  utilising  a  vacuum  valve  to  control  the  radiation  from
an  aerial  in  a  similar  manner  to  the  Alexanderson  Magnetic
Microphone.1  The  valve  is  arranged  in  shunt  to  the  aerial
circuit  or  to  the  oscillation  generator  (alternator,  arc,  valves,


FIG.  198. — Shunt  Valve  Microphonic  Control  with  Double  Anode  Valve.
etc.),  so  that  a  variable  portion  of  the  energy  that  would
normally  be  radiated  is  diverted  from  the  aerial  circuit.  One
arrangement  is  shown  in  Fig.  197.  The  source  of  H.F.
