Fields  by  means  of  an  Oscillating  Electric  Needle.       405 
further  to  investigate  the  nature  of  the  disturbance  caused, 
and  to  determine  its  bearing  on  the  applicability  of  the  needle 
for  exact  quantitative  measurements  of  field. 
A  case  of  importance  is  that  of  the  field  between  two 
parallel  metal  plates.  The  question  may  be  put  thus :  if  an 
electric  needle  be  placed  in  an  infinite  uniform  field,  at  what 
distance  does  the  disturbing  influence  of  the  needle  cease 
to  be  appreciable  ?  I  know  of  no  mathematical  treatment 
for  the  case  of  a  short  cylindrical  needle  such  as  was  used 
above.  The  following  experiment  was  therefore  performed. 
The  parallel  plates  already  described  were  used,  and  the 
arrangement  of  apparatus  was  as  in  fig.  1.  The  plates  could 
be  moved  perpendicular  to  themselves  so  as  to  alter  their 
distance  d  apart.  A  constant  potential-difference  of  400 
volts  was  maintained  between  them.  The  needle  used  was  of 
aluminium,  of  the  following  dimensions  : — 
and 
length     =1*526  cms., 
diameter  =   '101     ,, 
=   *0329  grams. 
mass 
The  distance  d  was  varied  between  the  limits  of  6*04  cms. 
and  1*92  cms.,,  and  values  of  N"  for  these  and  several  inter- 
mediate values  of  d  were  determined.  The  needle,  suspended 
by  a  quartz  fibre,  was  set  centrally  between  the  plates  in 
each  case.     The  results  may  be  tabulated  as  follows  : — 
T0  =  18-80  sees. 
No.  of 
exp. 
d 
in  cms. 
T 
in  sees. 
per  sec. 
N 
=  VN'a-]V 
Nx^. 
1   
2  
3  
4  
5  
6  
7  
8  
9  
10  
6-04 
5-33 
4-82 
4-40 
4-03 
3-00 
2-53 
2-325 
2-07 
192 
720 
6-40 
5-82 
5-34 
5-00 
3-69 
3-04 
2-72 
2-32 
2-04 
•1392 
•1562 
•172 
•1866 
•200 
•271 
•329 
•368 
•433 
•490 
•1282 
•1468 
•163 
•178 
•193 
•2656 
•325 
•364 
•430 
•487 
■775 
•784       j 
•785 
•783 
•778 
•797 
•822 
•844 
•885 
•916 
These  numbers  are  represented  graphically  in  fig.  2,  where 
products  of  frequency  x  distance  apart  of  plates  (Nxd)  are 
plotted  against  distance  apart  of  plates  (d).  From  this 
curve  it  may  be  seen  that  beyond  d  =  ±-5  cms.,  or  say 
exceeding  three  times  the  length  of  the  needle,  the  disturbing 
