Fields  by  means  of  an  Oscillating  Electric  Needle.        409 
Where  the  disturbance  is  not  of  importance,  and  it  is  desired 
to  show  the  vibrations,  either  directly  or  by  projection  in  a 
lantern,  the  needle  may  with  advantage  be  large.  Aluminium 
needles  of  1J  cms.  length  and  1  mm.  diameter  have  been 
found  very  satisfactory  ;  brass  needles  should  be  of  smaller 
diameter.  For  strong  fields  the  needles  may  be  still  heavier, 
and  the  suspension  maybe  of  silk  threads  consisting  of  several 
single-fibres.  There  is  thus  little  danger  of  breakage  of  the 
suspension,  where,  as  for  purposes  of  lecture  demonstration, 
the  needle  has  to  be  moved  about  and  may  be  more  or  less 
roughly  used. 
Applications. 
The  electric  needle  may  be  regarded  as  a  measurer  of 
electric  force,  and  be  applied  to  test  the  field-strength  due  to 
any  arrangement  of  conductors,  effect  of  alteration  of  their 
relative  position,  or  of  the  potential,  &c.  In  some  cases 
exact  results  will  not  be  obtained  owing  to  the  disturbance 
introduced  by  the  needle  itself.  In  others  the  disturbing 
effect  of  the  needle  does  not  enter.  Of  the  first  class  the 
following  example  may  be  cited  : — 
Comparison  of  the  electric  field  at  different  points  on  the 
surface  of  any  conductor. 
The  case  of  a  pear-shaped  conductor  (fig.  4)  was  examined. 
Fijv  4. 
Iquater. 
Four  positions  on  it  were  chosen,  namely,  at  the  pointed  end 
and  blunt  end,  at  the  equator,  and  opposite  the  middle  of 
the  flat  side  (where  the  curvature  is  single) .  The  needle 
used  was  H  cms.  long.  Its  nearer  end  was  set  in  each  case 
Phil,  Mag.  S.  6.  Vol.  11,  No.  63,  March  1906.      2  E 
