734  Prof.  J.  S.  Townsend  on  the  Field  of  Force 
easily.  The  upper  ends  of  the  rods  were  screwed  to  a  piece 
of  brass  X  having  a  hole  in  the  centre  through  which  the 
upper  portion  of  tlr*  axle  S  passed  freely  without  touching. 
The  wire  W,  the  brass  piece  X,  and  the  two  rods  R  thus 
formed  a  rigid  rectangular  frame  which  could  be  moved  up 
or  down,  and  the  brass  tubes  T  exerted  sufficient  pressure  on 
the  ebonite  rods  to  keep  the  frame  in  any  position  in  which 
it  might  be  placed.  In  order  to  raise  the  frame  the  screw 
S  was  rotated  until  the  disk  D  moved  the  brass  piece  X  to 
the  required  position.  The  frame  was  lowered  by  screwing 
down  the  axle  so  as  to  make  the  short  piece  of  ebonite  G  on 
the  top  of  the  axle  to  press  on  the  upper  side  of  X.  It  was 
arranged  that  the  wire  W  should  be  one  millimetre  below  the 
plate  B  when  the  ebonite  G  was  pressing  on  X.  When  the 
distance  between  the  plates  A  and  B  did  not  exceed 
8  millimetres  it  was  possible  to  place  the  wire  accurately  at 
any  distance  from  the  lower  plate,  except  that  it  could  not  be 
brought  nearer  than  one  millimetre  to  the  upper  plate. 
Electric  connexion  was  made  with  the  upper  plate  and 
the  wire  through  ebonite  plugs  in  the  side  tubes  of  the  glass 
cover,  and  the  connexion  with  the  lower  plate  was  made  by 
the  base. 
A  sensitive  electrostatic  voltmeter  which  gave  readings  to 
