ELECTRICAL  DISCHARGES  AS  OBSERVED  IN  TORRICELLIAN  VACUA. 
13 
far  more  brilliant  on  platinum  wire  after  the  discharge  has  been  made  for  a short  time,  pro- 
bably from  the  incandescence  of  the  minute  points  of  the  metal  in  the  act  of  emanation 
as  they  are  deposited  on  the  glass ; if  a very  thin  platinum  wire  is  used  for  the  negative 
terminals,  it  will  be  fused.  I have  already  mentioned,  that  in  the  ball  apparatus  (fig.  G) 
there  was  not  any  discharge  or  emanation  of  particles  of  mercury  from  the  positive  ball, 
but  much  from  the  negative,  another  evidence  that  the  force  from  the  positive  is  not 
accompanied  by  the  transfer  of  particles  of  the  metallic  terminal. 
Action  of  a Magnet  on  the  Inductive  Discharge. 
44.  The  stratifications  are  very  powerfully  affected  by  a magnet.  When  the  discharge 
is  made  from  wire  to  wire,  Plate  I.  figs.  1,  2,  3 or  7,  if  a horseshoe  magnet  is  passed 
along  the  tube  so  as  alternately  to  present  the  poles  to  different  contiguous  positions 
of  the  discharge,  it  will  assume  the  form  of  ~ in  consequence  of  its  tendency  to  rotate 
round  the  poles  in  opposite  directions,  as  the  magnet  in  this  position  is  moved  up  and 
down  the  side  of  the  tube. 
The  effect  is  still  more  striking  if  the  straight  bar  of  a powerful  electro-magnet  is 
placed  close  to  the  ends  of  the  stratifications ; they  then  tend  to  rotate  in  one  direction 
round  the  north,  and  in  another  round  the  south  pole  of  the  magnet. 
45.  If  the  mercury  in  apparatus  (28.),  fig.  7,  is  allowed  to  ascend  so  as  to  cover  the 
negative  whe,  and  thus  produce  a mercurial  negative  terminal,  the  action  of  the  mag- 
net on  the  stratifications  mil  be  found  to  be  much  heightened ; as  soon  as  the  electro- 
magnet is  excited,  whether  the  polarity  is  north  or  south,  the  stratifications  from  the 
positive  are  violently  drawn  down  the  tube  as  an  elongated  spiral ; under  certain  con- 
ditions they  will  assume  the  appearance  of  rotation,  and  if  the  magnet  is  such  as  to 
present  north  polarity  to  the  discharge,  the  rotation  is  from  right  to  left ; if  south,  from 
left  to  right.  I am  aware  that  there  is  a difference  of  opinion  as  to  the  actual  rotation 
of  these  bands,  and  that  in  many  cases  there  is  no  evidence  or  even  appearance  of  rota- 
tion ; but  under  the  influence  of  the  magnet,  as  thus  arranged,  there  cannot  be  any  doubt 
as  to  the  appearance  being  such  as  I have  described.  On  the  negative  glow  in  this 
apparatus  the  magnet  appears  to  have,  in  comparison  with  that  on  the  positive  discharge, 
a very  slight  effect,  but  it  attracts  or  repels  it  according  to  the  polarity  of  the  magnet ; 
it  does  not  influence  or  produce  any  signs  of  stratification  in  the  negative  discharge. 
46.  When  the  discharge  was  first  made  in  the  pear-shaped  apparatus,  fig.  5 (24.),  the 
mercury  being  negative  and  about  2 inches  from  the  end  of  the  positive  wire,  the  dis- 
charge formed  nearly  a straight  line ; in  this  position,  when  the  pole  of  a powerful 
electro-magnet  was  placed  close  to  the  glass  vessel  of  the  apparatus,  the  discharge  was 
deflected  across  the  pole  at  right  angles,  the  discharge  being  from  the  positive  wire  to 
the  negative  mercury;  if  the  magnet  presented  a northern  polarity,  the  discharge 
deflected  to  the  right,  when  looking  from  the  magnet  to  the  discharge,  carrying  with  it 
the  red  spot  in  a direct  line  across  the  mercury. 
47.  When  the  discharge  was  made  in  the  apparatus  (fig.  6),  the  upper  ball  being 
