Fields  by  means  of  an  Oscillating  Electric  Needle.        411 
not  easy  to  demonstrate  it  experimentally.  In  the  electric 
needle  we  have  a  means  o£  doing  this  of  a  simplicity  corre- 
sponding to  that  which  the  proof-plane  affords  for  showing 
that  the  charge  inside  the  conductor  is  zero. 
As  the  needle  is  lowered  into  the  conductor,  e.  g,  a  metal 
can  or  a  cylinder  of  wire  netting,  the  frequency  of  the  needle 
falls  until  when  well  inside  its  value  is  the  same  as  when  the 
conductor  is  discharged. 
On  the  Shielding  Effect  of  Dielectrics. 
Having  mounted  a  needle  within  a  glass  specimen-tube  in 
order  to  do  away  with  the  effects  of  air-currents,  it  was 
observed  that  even  when  in  the  strongest  electric  fields  the 
needle  was  quite  unaffected.  The  tube  acted  as  if  it  were  a 
conductor.  As  this  effect  might  be  due  simply  to  the 
unclean  condition  of  the  surfaces,  the  experiment  was  repeated 
with  a  carefully  cleansed  glass  lamp-chimney,  with  the  same 
result.  It  appears  then,  that  owing  to  the  conductivity  of 
the  glass,  slight  as  it  is,  a  closed  glass  vessel  screens  electric 
force  from  its  interior,  just  as  if  it  were  a  conductor. 
The  same  result  was  found  for  a  sheet  of  mica  of  thickness 
jL  mm<  bent  into  a  cylinder.  A  mica  cylinder  in  ordinary 
air  screens  the  interior  completely  (when  placed  in  a  steady 
field,  as  in  fig.  5).  If  heated  over  a  hot  plate  and  quickly 
transferred  to  the  field  so  as  to  surround  a  needle  swinging 
in  it,  there  is  seen  to  be  electric  force  at  the  needle  ;  but  it 
quickly  dies  down,  and  in  20  or  30  seconds  the  needle  once 
more  vibrates  in  its  own  natural  period,  depending  on  the 
torsional  couple  of  the  quartz  suspension. 
This  conductivity  of  the  mica  appears  to  be  a  surface 
effect,  probably  due  to  the  deposition  of  moisture  from  the 
atmosphere.  For  if  the  mica  sheet  be  immersed  for  some 
time  in  melted  paraffin-wax  at  a  temperature  close  on  200°  C, 
then  taken  out  and  allowed  to  cool,  the  sheet  (made  into  a 
cylinder  as  before)  is  found  to  be  capable  of  transmitting  the 
field  for  some  time.  But  even  in  this  case  the  field  within 
the  cylinder  falls  off  with  time.  Thus  in  one  case  a  needle 
within  a  paraffined  mica  cylinder  gave  12  vibrations  in 
10  seconds  at  first,  but  four  minutes  later  only  8  vibrations 
were  counted  in  the  same  time,  the  fall  of  frequency  being- 
according  to  the  exponential  law.  Electrical  separation 
has  slowly  taken  place  in  the  mica.  If  now  the  plates 
between  which  the  field  was  produced  be  discharged,  the 
cylinder  should  be  left  charged,  and  an  electric  field  should 
exist  in  the  interior.  This  is  found  to  be  the  case.  The 
field  dies  out  with  time,  as  in  the  experiments  in  the  constant 
2E  2 
