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LXXVII.   An  Experiment  with  the  Electric  Arc. 
B\j  A.  A.  Campbell  Swinton*. 
IX  a  recent  communication  to  the  Royal  Society  (Proc. 
Royal  Society,  A.  vol.  lxxvi.),  the  writer  described  an 
experiment  on  the  electric  arc  somewhat  similar  to  that 
adopted  by  Perrin  (Comptes  Rendus,  vol.  cxxi.  p.  1130,  1895) 
for  demonstrating  the  electric  charge  carried  by  cathode  rays. 
In  the  writer's  experiment  one  of  the  carbon  electrodes  was 
made  hollow  and  was  pierced  axially  by  a  small  hole,  an 
insulated  Faraday  cylinder  of  brass,  with  its  aperture  in  line 
with  and  facing  the  aperture  in  the  electrode,  being  fixed 
inside  the  electrode.  The  insulated  Faraday  cylinder  and 
the  pierced  electrode  were  connected  together  through  a 
galvanometer,  and  it  was  found  that  when  the  opposite 
electrode  was  made  neqative,  the  pierced  electrode  being- 
positive,  a  negative  charge  was  imparted  to  the  Faraday 
cylinder,  while,  if  the  opposite  electrode  was  made  positive 
and  the  pierced  electrode  was  made  negative,  the  Faraday 
cylinder  acquired  a  positive  charge. 
These  results  could  be  obtained  in  air  at  atmospheric 
pressure,  but  more  easily  obtained  in  a  partial  vacuum. 
Following  on  this  application  of  Perrin's  method  to  the 
electric  arc,  the  writer  has  made  further  investigations  with 
a  view  to  ascertaining  whether,  with  the  arc,  any  effect  could 
be  obtained  analogous  to  that  found  by  Lenard  (Wied.  Ann. 
pp.  51,  225,  1891),  who  discovered  that  cathode  rays  would 
pass  through,  or  at  any  rate  produce  effects  as  though  they 
passed  through,  a  diaphragm  of  very  thin  aluminium-foil. 
Part  of  the  apparatus  is  illustrated  full  size  in  section  in 
fig.  1.  "  A "  is  the  hollow  carbon  electrode  pierced  by  a 
small  aperture  about  3  mm.  diameter  at  its  lower  extremity; 
"  B  "  is  the  opposite  carbon  electrode,  while  "  C  "  is  the  brass 
Faraday  cylinder.  As  the  carbons  rapidly  attain  a  tem- 
perature above  the  melting-point  of  aluminium,  the  diaphragm 
of  the  latter  metal  "  D " — believed  to  be  about  the  same 
thickness  as  that  used  by  Lenard,  i.  <?,  "00265  mm. — was  kept 
out  of  direct  contact  with  the  carbon  by  being  fixed  over  a 
small  aperture  in  a  brass  cylinder  "  E,"  which  was  in  contact 
with  the  upper  carbon  at  a  point  where  the  latter  was 
comparatively  cool.  "A,"  "  Y>"  and  "  E  '*  were  all  in  good 
electrical  connexion  and  were  connected  to  earth,  while  the 
Faraday  cylinder  was  connected  to  a  quadrant  electrometer, 
The  experiments  were  tried  both  in  air  at  ordinary  atmo- 
spheric pressure,  and  also  in  vacua  up  to  about  750  mm.  of 
mercury. 
*  Communicated  bv  tho  Author. 
