Tube,  Pliicker  — : A  modification  of  a

Geissler  tube  adapted  for  the  study  of  the
stratification  of  the  light,  and  the  peculiar-
ities of  the  space  adjoining  the  negative  elec-
trode. (See  Tubes,  Geissler)


Tube,  Spark


— A  high  vacuum  tube,


across  which,  when  the  vacuum  is  sufficiently
high,  the  spark  from  an  induction  coil  will  not
pass.
A  spark  tube,  connected  with  incandescent
lamps  while  undergoing  exhaustion,  acts  as  a
simple  gauge  to  determine  the  degree  of  ex-
haustion. When  an  induction  coil  discharge
ceases  either  to  pass,  or  to  pass  freely,  the  vacuum
is  considered  as  sufficient,  according  to  circum-
stances.
Tube,  Stratification An  exhausted

glass  tube,  the  residual  atmosphere  of  which
displays  alternate  dark  and  light  stria',  or
stratifications,  on  the  passage  through  it  of
an  induction  coil  discharge.  (See  Discharge,
Luminous  Effects  of)

Tubes,  Geissler Vacuum  tubes  of

glass  containing  various  gases,  liquids  or
solids,  provided  with  platinum  electrodes,
passed  through  and  fused  into  the  glass,  de-
signed to  show  the  various  luminous  effects


Tub.]


535


[Twi.
of  electric  discharges  through  gases  at  com-
paratively low  pressures.
Geissler  tubes  are  made  of  a  great  variety  of
shapes,  and  often  include  tubes,  spirals,  spheres,
etc.,  within  other  tubes.  These  enclosed  tubes
are  made  either  of  ordinary  glass,  or  of  uranium
glass  in  order  to  obtain  the  effects  of  fluorescence.
The  vacuum  in  Geissler  tubes  is  by  no  means
what  might  be  called  a  high  vacuum.  Indeed,  if
the  exhaustion  of  the  tube  be  pushed  too  far,
much  of  the  brilliancy  of  the  luminous  effects  is
lost.
Some  of  the  many  forms  of  Geissler  tubes  are
shown  in  Fig.  560.
Fig.  360.     Geisder  Tubes.
