410  Mr.  D.  Owen  on  the  Comparison  of  Electric 
at  a  distance  from  the  conductor  equal  to  half  the  length  of 
needle.     The  results  were  as  follows  : — 
Position. 
Time  of  oscillation 
T  (sees.). 
Relative  frequency. 
Pointed  end  
2-55 
3  20 
325 
3-85 
1 
•8 
•78 
•66 
Blunt  end 
Equator 
1 
For   a  couple  of  spherical  conductors  at  a  distance  from 
other  conductors  the  following  numbers  were  obtained  : — 
Diameter  of 
sphere. 
T. 
Relative 
frequencies. 
Electric  force 
(theoretical). 
12  cms. 
3     „ 
2-40  sees. 
1-23     „ 
1 
1-96 
1 
4 
These  comparative  numbers  indicate  the  increase  of  electric 
force  F  with  curvature.  By  means  of  a  proof-plane  the 
increase  of  surface-density  or  of  charge  with  curvature  may 
be  shown.  In  both  cases  the  numbers  suffer  in  accuracy 
owing  to  the  disturbance  introduced  by  test  needle  and  proof- 
plane  respectively. 
Such  experiments  as  the  above  are  best  performed  by 
attaching  the  conductor  by  a  fine  wire  to  the  inner  coating 
of  a  charged  Leyden  jar.  The  potential  then  changes  only 
very  slowly  with  time. 
It  may  be  interesting  to  add  that  the  potential  of  the  pear- 
shaped  conductor,  as  determined  by  an  electric  needle,  was 
6200  volts  in  the  case  to  which  the  numbers  above  refer. 
In  the  following  two  cases  the  disturbing  effect  of  the 
needle  is  of  no  significance  : — 
(1)  Wherever  the  variation  of  electric  force  with  altera- 
tion of  potential  is  to  be  found,  no  motion  of  the  bounding 
conductors  or  of  position  of  needle  taking  place.  The  electric 
force  as  measured  by  the  frequency  of  the  needle  is  propor- 
tional to  the  difference  of  the  potential. 
(2)  To  prove  that  inside  a  charged  conductor  (not  enclosing 
an  insulated  charged  conductor)  the  electric  force  is  zero. 
Though  this  theorem  is  one  of  fundamental  importance,  it  is 
