VACUUM  OSCILLATION  GENERATORS         165


oscillations  of  fairly  definite  frequency  may  usually  be  set  up
in  this  manner,  just  as  in  the  case  of  the  oscillating  arcs.1

The  occurrence  of  this  negative  characteristic  in  vacuum

valves     is     usually     most

evident     with    very    high
vacua.2

In  practice  it  is  fre-
quently the  custom  to  sur-
round the  filament  loop  *
with  a  metallic  cylinder  y
instead  of  using  merely  a
small  plate  between  the
legs.  Such  arrangements
lower  the  resistance  of  the
valve  and  allow  larger  cur-
rents to  be  passed  through
it.  At  the  same  time  the
cylinder  serves  to  shield
the  glass  bulb  from  static
charges.  This  factor  is

often  of  considerable   im-      0^™?£%£gS££&r.
portance,  as  will  be  evident

later.  Fig  118  is  a  diagram  of  this  type  of  construction.
Very  many  forms  and  types  of  valves  have  been  brought
out  at  various  times.  Some  of  these  are  dealt  with  in  the
following  pages.
A  piece  of  apparatus  in  which  a  discharge  in  rarefied  gas
shows  a  negative  characteristic  is  the  tungsten  arc  in  an
inert  gas.  The  commercial  form  of  this  is  usually  known  as
the  "  Pointolite  "  lamp.3  The  characteristic  curve  of  these
lamps  is  very  steep  for  small  currents  and  high  voltages,  but
becomes  very  much  natter  at  the  normal  current  for  which
the  lamps  are  rated.
Oscillations  may  be  obtained  by  the  use  of  one  of  these
arcs  in  hydrogen,  shunted  by  a  usual  oscillation  circuit.4

MODE  OF  OPERATION  OF  VACUUM  VALVES  AND

OSCILLATION  GENERATORS

The  construction  of  a  Fleming  vacuum  oscillation  valve
is  indicated  in  the  diagram  of  Fig.  118.     G  is  the  sealed  glass
1  Reference  No.  309.  2  Reference  No.  308.
3  Reference  No.  310.  4  Reference  No.  351.
