ELECTEICAL  DISCHAEGES  AS  OBSERVED  IN  TORRICELLIAN  VACUA. 
9 
30.  Let  the  terminal  of  one  wire  be  firmly  fixed  to  a bright  copper  plate,  -g^th  of  an  inch 
thick,  having  a sharp  edge  in  order  to  ensure  a sudden  disruption  of  the  primary  cur- 
rent ; complete  the  circuit  by  steadily  pressing  the  end  of  the  other  wire  on  the  plate, 
using  one  or  two  of  Grove’s  cells  to  excite  the  primary : no  trace  of  any  discharge  will 
be  perceived  in  the  tube. 
Pressing  the  wire  on  the  plate  so  as  to  ensure  perfect  continuity,  bring  it  across 
the  sharp  edge  of  the  copper  plate,  and  a quick,  sudden  break  of  the  battery  circuit  can 
be  effected ; the  stratifications  will  appear  in  the  tube  in  a very  distinct  and  beautiful 
manner;  in  a discharge  of  18  inches  the  entire  tube  is  filled  with  them  to  the  extent  of 
the  dark  band  (36.);  the  more  sudden  the  break,  the  more  distinct  will  be  the  effects. 
If  the  experiment  is  repeated  by  making  contact  with  the  wires  without  the  plate,  or 
by  dipping  them  in  the  mercury,  the  same  results  are  obtained.  I have  made  the  expe- 
riment of  a smgle  disruption  of  the  primary  circuit  under  every  variety  of  form,  and  in 
no  instance  did  it  fail,  although  unless  care  is  taken,  it  might  be  fairly  doubted  whether 
the  discharge  was  single  or  continuous. 
31.  V^Tien  the  intensity  of  the  primary  circuit  is  increased  by  using  eight  or  ten  of 
Grove’s  battery,  the  stratification  is  plainly  visible  and  on  making  contact^  but  in  an 
opposite  direction  and  very  inferior  in  intensity  or  brilliancy  to  the  discharge  on  breaking ; 
the  effects  are  much  heightened  by  the  condenser. 
32.  Although  no  signs  of  any  discharge  can  be  observed  in  the  vacuum  on  making 
contact  when  the  primary  current  is  excited  by  a single  cell,  still  evidence  of  action  in  the 
secondary  coil  can  be  detected  by  the  galvanometer,  as  also  in  the  following  manner : — 
let  the  ends  of  tw'o  platinum  wires  attached  to  the  terminals  of  the  secondary  coil  rest 
on  a piece  of  bibulous  paper,  moistened  with  a solution  of  iodide  of  potassium ; complete 
the  circuit  of  the  commutator,  and  then  remove  the  paper : iodine  will  be  evolved  from 
one  terminal: — make  contact  with  the  moistened  paper,  keeping  the  primary  circuit 
complete ; remove  the  paper ; no  trace  of  iodine  is  perceivable,  proving  that  in  this 
state  there  is  no  e\idence  of  a current  in  the  secondary  wire : again  make  contact  with 
the  moistened  paper  and  the  platinum  wire,  break  the  circuit  with  the  commutator,  and 
iodine  is  immediately  evolved  at  the  opposite  terminal  in  a much  larger  quantity  than 
the  former. 
33.  In  the  phenomena  of  the  induction  spark,  time  appears  to  be  an  important  element, 
at  least  it  is  so  in  the  secondary  coil  aS'  at  present  constructed ; experiment  teaches  us 
that  the  effect  from  breaking  is  far  more  powerful  than  that  from  making  contact.  In 
Ruhmkorff’s  coils  we  have,  however,  not  only  the  induced  efiect  from  the  primary  cur- 
rent, but  we  have  the  induced  effect  from  the  powerful  electro-magnet;  to  attain  the 
maximum  effect  of  the  latter  time  is  ^ particularly  if  the  primary  coil  is  formed 
of  several  convolutions  of  the  wire  (“three  convolutions  are  used  by  Ruhmkorff”). 
* Time,  as  is  well  known,  is  necessary  for  the  development  of  electro-magnetism,  and  M.  Matteucci,  in 
his  recent  valuable  book  on  induced  electricity,  has  shown  some  remarkable  results  flowing  from  this  fact. 
— Grove,  Philosophical  Magazine,  January  1855,  p.  2. 
MDCCCLVIII.  C 
