Dielectric  Strength  of  Air. 
269 
with  the  electrodes  used  by  Jona,  were  the  same  in  the 
two  cases.  For  instance,  when  the  maximum  value  of  the 
applied  P.D.  was  240  kilovolts  (—  =  42),  the  sparking- 
distance  was  47  cms.,  whether  the  point  and  the  plate  or  the 
two  spherical  electrodes  were  used.  Faraday  anticipated  this 
result  in  his  '  Experimental  Researches,'  §  1499  : — '*'  But 
as  has  long  been  recognized,  the  small  body  is  only  a 
blunt  end,  and,  electrically  speaking,  a  point  only  a  small 
ball ;  so  that  when  a  point  or  blunt  end  is  throwing  out  its 
brushes  into  the  air,  it  is  acting  exactly  as  the  small  balls 
have  acted  in  the  experiments  already  described,  and  by 
virtue  of  the  same  properties  and  relations/'' 
(v.)   C.  P.  Steinmetz.     0'795  cm.  cylindrical  electrodes 
(a=0-3975). 
Professor  Steinmetz  in  the  paper  referred  to  above  also 
describes  tests  on  the  sparking-distances  between  cylindrical 
electrodes.  In  one  case  the  electrodes  were  two  copper  rods 
0*795  cm.  in  diameter  and  71  cms.  long.  The  rods  were 
slightly  curved,  so  that  the  sparks  ensued  across  the  minimum 
distance  between  them.  The  radius  of  curvature  at  this  part 
was  198  cms.,  and  so,  provided  the  rods  are  not  further 
apart  than  about  2  cms.,  we  can  neglect  the  curvature  and 
assume  that  the  field  is  very  similar  to  that  between  two 
infinitely  long  parallel  straight  rods.  We  can  therefore  use 
the  formula  (27)  for /'given  in  §  11. 
Table  X. 
C.  P.  Steinmetz.     Two  cylindrical  rods  slightly  carved. 
Diameter  of  rods  6-7?5"cm.     —  =  125,  E/V  =  P42. 
JSo.  of 
Experiment. 
1   
X. 
0-165 
0-203 
0-433 
0-881 
1-115 
1-194 
1-435 
1-753 
2-134 
2-3G2 
xja. 
/(calc). 
V  (obs.). 
Emax. 
0-42 
0-51 
115 
2-21 
2-81 
301 
3-61 
4-40 
5-38 
5-96 
1-07 
1-08 
1-18 
1-35 
1-43 
1-46 
1-55 
1-66 
1-80 
1-88 
4-78 
5-70 
10-85 
18-55 
23-7 
22-9 
23-0 
30-5 
34-3 
33-1 
38-9 
38-8 
38-9 
391 
42-2 
3S-8 
34  3 
40-3 
40-4 
368 
2  
3  
4 
5  
6  
8  
9  
10 
The  mean  of  the   values  of  Pmax.  given  in  the  last  column 
makes   the    dielectric    strength    of 
centimetre. 
ai 
r    38-8  kilovolti 
pei 
