402  3Ir.  D.  Owen  on  the  Comparison  of  Electric 
time  owing  to  the  interference  of  too  many  sources  of  error. 
With  our  instrument  we  arrange  that  a  contact  should  be 
made  by  the  dynamo,  gas-engine,  motor  car,  &c.  &c,  at 
convenient  intervals,  say  each  1  or  10  or  100  revolutions. 
The  distances  between  the  contacts  of  the  spiral  on  paper 
ruled  in  mm.  to  indicate  the  time  gives  also  at  a  glance  the 
character  of  the  varying  speed,  but  the  same  unit  divided  by 
the  distances  between  two  contacts  read  in  mm.,  gives  at  the 
same  time  also  exactly  the  speed  between  the  time  intervals 
indicated  on  the  paper.  The  greater  length  of  spiral  also 
allows  of  this  variation  being  observed  more  frequently  and 
with  greater  accuracy. 
London,  August  1905. 
XXXIII.  The  Comparison  of  Electric  Fields  by  means  of 
an  Oscillating  Electric  Seedle.  By  David  Owex,  B.A. 
(Camb.),  B.Sc.(Lond.),  Lecturer  in  Physics,  Birkbeck 
College,  London*. 
EXPERIMENTS  on  fields  of  force  due  to  electric  charges 
suffer  as  compared  with  the  corresponding  measure- 
ments in  magnetic  fields  by  reason  of  the  impracticability  of 
obtaining  an  lt electric  needle"  corresponding  to  the  oscillating 
magnetic  needle.  But  while  it  is  true  that  a  permanently 
electrified  needle  cannot  be  obtained,  it  is  possible  to  have 
one  that  has  equal  charges  of  opposite  sign  induced  by  the 
field.  This  paper  contains  an  account  of  experiments  on 
the  use  of  such  a  needle  for  the  measurement  of  electric 
rields,  steady  or  alternating,  and  for  the  experimental 
illustration  of  some  of  the  laws  of  electrostatics. 
Theory. 
Consider  a  cylindrical  needle  supported  at  its  centre  by  a 
fibre  whose  torsional  control  is  negligible,  placed  with  its 
axis  horizontal  in  a  uniform  horizontal  field  of  strength  F. 
The  needle  will  move  so  as  to  set  itself  parallel  to  the  field. 
If  disturbed  from  that  position  a  restoring  couple  will  come 
into  play.  This  couple  will  be  proportional,  both  to  the 
induced  charge,  i.  e.  to  the  "  pole-strength,r  and  to  the  field. 
As  the  former  is  proportional  to  the  latter,  it  follows  that 
the  couple  will  vary  as  the  square  of  the  field-strength.  It 
will  diminish  as  the  angle  between  the  axis  of  the  needle 
and  the  direction  of  the  field  increases,  and  will  clearly  be 
zero  when  the  two  are  at  right  angles.     In  the  case  of  an 
*  Communicated  by  the  Physical  Society  :  read  June  30, 1905. 
