It  may  be  mentioned  in  passing  that  the  generation  of
continuous  electric  oscillations  in  the  above  manner  is  not
limited  to  electric  arcs.  Any  conductor  having  a  negative
or  "  falling  "  characteristic  may  be  employed  for  the  same
purpose.  For  instance,  certain  metallic  oxides  have  such  a
characteristic  and  can  be  caused  to  set  up  oscillations  in
associated  circuits.  A  Nernst  lamp-glower  will  show  this
effect.5  Other  examples  are  mentioned  on  pp.  164  and  165.
The  frequency  of  the  oscillations  that  are  obtained  may  be
limited  by  other  considerations  (such,  for  example,  as
"  hysteresis  "  6),  but  given  suitable  circuits,  oscillations  of
certain  frequencies  can  be  set  up.
Experiment  shows  that  when  an  arc  is  functioning  in  the

1  See  also  Reference  No.  206.
2  Reference  Nos.  43,  202  ;   see  also  Nos.  203,  204.
3  Also  Reference  Nos.  200,  201.  4  Reference  No.  210.
8  Reference  No.  209.  6  See  below.
ARC  OSCILLATION  GENERATORS


115


manner  set  out  above,  its  characteristic  curve  no  longer
retains  precisely  the  shape  indicated  in  Fig.  77,  but  that  there
is  a  certain  so-called  "  hysteresis  "  effect,  as  a  result  of  which
the  characteristic  does  not  follow  the  same  curve  for  decreas-
ing currents  as  it  does  for  increasing  ones.1  Fig.  79  illustrates
diagrarnmatically  one  form  taken  by  the  curve  under  these
conditions,  between  current  limits  Ia  and  I2,  the  arrows
indicating  the  parts  of  the  curve  for  increasing  and  decreasing
currents  respectively.  From  the  form  of  the  curve  it  is
evident  that  the  effect  is  due  to  the  changes  in  temperature


,6


FIG.  79. — Dynamic  Characteristic  of  Arc.
CD = Static  Characteristic  for  the  same  Arc.
