404  Mr.  D.  Owen  on  the  Comparison  of  Electric 
The  apparatus  was  enclosed  in  a  balance-case  in  order  to 
avoid  draughts.  Observations  were  restricted  within  an 
amplitude  of  oscillation  of  about  5°,  in  which  case  the  error 
due  to  finite  amplitude  is  negligible. 
J?eadi;-vg 
JeZesccTpe-. 
The  needle  used  was  of  copper  wire,  1*4  cms.  long  and 
2  mms.  in  diameter,  suspended  by  a  quartz  fibre  about  one- 
tenth  of  a  mm.  in  diameter  and  8  centimetres  long.  The 
plates  were  at  a  distance  of  3  cms.  apart.  The  results  gave 
as  expected  a  straight-line  law  connecting  V  and  K". 
Disturbance  of  the  Field  due  to  the  presence  of  the  Needle. 
The  effect  of  the  presence  of  such  a  needle  in  an  electric 
field  is  in  every  case  to  produce  a  concentration  of  the  field 
near  the  needle.  Further,  if  the  charged  conductor  forming- 
one  of  the  boundaries  of  the  field  is  sufficiently  near  the 
needle,  the  distribution  of  the  charge  on  that  boundary  will 
be  altered.  This  of  course  prevents  the  application  of  the 
electric  needle  for  the  accurate  comparison  of  the  electric 
forces  near  the  surfaces  of  conductors  of  different  forms  and 
curvatures.     It  was  considered  desirable  before  proceeding 
