412  Mr.  D.  Owen  on  the  Comparison  of  Electric 
field.  Another  way  of  showing  this  effect  of  minute  con- 
ductivity is  to  introduce  the  cylinder  into  a  steady  field, 
leave  it  there  for  some  time,  and  then  turn  it  about  its  axis 
through  180°.  The  field  inside  the  cylinder  is  seen  to  be  for 
the  moment  greater  than  that  of  the  original  field,  for  on  the 
latter  is  superposed  the  field  due  to  the  separated  charges  on 
the  walls  of  the  cylinder.  With  cylinders  of  thicker  sheets 
of  mica  similarly  treated  with  paraffin -wax,  the  rate  of 
falling  off  of  the  interior  field  is  found  to  be  greater.  These 
experiments  indicate  an  appreciable  though  exceedingly 
small  conductivity  in  the  mica  itself. 
Experiments  on  the  shielding  effect  of  ordinary  white 
paper  were  made.  Cylinders  of  paper  screen  the  interior 
completely  from  the  surrounding  field.  But  on  heating  for 
a  short  time  over  a  hot  plate  the  shielding  effect  vanishes  ; 
in  about  one  minute,  however,  the  frequency  of  the  needle 
within  is  found  to  have  fallen  considerably,  and  a  minute 
later  is  the  same  practically  as  in  zero  field.  Dry  paper 
then  acts  as  a  perfect  insulator.  But  by  the  absorption  of 
moisture  when  exposed  to  the  open  air  it  quickly  gains 
appreciable  conductivity. 
With  paraffined  paper  the  shielding  power  wTas  nil,  and 
remained  nil  for  days  in  the  open  air  of  the  room.  Of  all 
the  materials  tried  this  alone  permitted  for  any  length  of 
time  the  undiminished  transmission  of  electric  force.  Thus 
by  applying  a  thin  coating  of  paraffin  wrax  to  a  sheet  of 
dielectric,  and  then  observing  its  shielding  effect  on  an  electric 
needle,  we  have  a  convenient  means  of  testing  the  conductivity- 
of  the  dielectric,  which  removes  all  doubts  as  to  surface- 
action.  This  effect  of  the  high  insulating,  non-hygroscopic, 
quality  of  paraffin-v^ax  is  made  use  of  in  the  experiments  on 
mica  above  described. 
Experiments  with  alternating  fields. — As  with  steady  fields 
glass  and  mica  proved  sufficiently  conducting  to  screen  off 
electric  effects,  experiments  were  made  wTith  these  materials, 
using  alternating  electric  fields.  The  alternating  voltaoe 
was  obtained  by  the  use  of  the  Musical  Arc.  Mr.  Duddell, 
I  may  say  here,  very  kindly  advised  me  as  to  the  conditions 
for  obtaining  high  voltages  by  this  means.  The  connexions 
were  as  showm  in  fig.  5. 
The  alternating  voltage  on  the  self-induction  P  in  the 
shunt  circuit  to  the  arc  was  found  to  be  about  200  volts. 
This  was  transformed  up  by  means  of  a  second  coil  S,  to  the 
terminals  of  which  the  parallel  plates  were  connected.  In 
this  way  any  voltage  up  to  1000  volts  could  be  maintained 
between  the  plates.      The  frequency  of  alternation  of  this 
