Fields  by  means  of  an  Oscillating  Electric  Needle. 
41i 
voltage  was  found,  by  means  of  a  sonometer  and  standard 
fork,  to  be  1700  per  second.  The  needle  was  allowed  to 
vibrate  in  the  field  with 
(a)  glass  cylinder  surrounding  it  ; 
(6)  cylinder  taken  away. 
Fio;.  5. 
The  time  of  vibration  was  found  to  be  the  same  in  each 
case,  namely  1/83  sees.  The  glass  oylinder  had  a  diameter 
of  5  cms.,  the  thickness  of  wall  being  1*4  mm.  With  such 
dimensions  the  weakening  of  the  field  inside  the  cylinder  due 
purely  to  the  specific  inductive  capacity  effect  should  only 
be  small.  With  a  cylinder  of  thicker  glass,  but  diameter 
only  slightly  exceeding  the  length  of  the  needle  (1  cm.),  the 
frequency  of  vibration  was  distinctly  less  in  case  (a)  than  in 
case  (b).     With  mica  cylinders  the  same  result  was  obtained. 
These  experiments  emphasise  the  necessity  of  using  alter- 
nating fields  in  all  cases  where  from  the  measurement  of  the 
force  experienced  by  a  mass  of  a  dielectric  its  specific  in- 
ductive capacity  is  to  be  determined.  In  steady  fields,  unless 
the  specific  conductivity  of  the  material  be  quite  nil,  the 
force  will  ultimately  be  the  same  as  for  a  conductor.  This 
consideration  will  account  for  the  high  values  of  k  obtained 
in  some  of  the  experiments  of  Boltzmanu  (see  Gray's  Abs. 
Measmts.  vol.  i.  p.  465). 
Use  of  Electric  Needle  to  measure  lrolts. 
A  pair  of  insulated  parallel  metal  plates  with  a  needle 
suspended  centrally  between  them  forms  a  simple  means  of 
measuring  high  voltages.     For  we  have 
volts  across  plates= frequency  of  needle  x  a  constant. 
The  constant  may  be  determined  at  a  low  voltage  by  any 
voltmeter  available.  The  highest  voltage  measurable  is 
limited  by  the  difficulty  of  counting   the  frequency  of  the 
