Dielectric  Strength  of  Air. 
275 
value  of  Rmax.  is  38*5.  Both  of  these  results  agree  very 
closely  with  the  final  conclusions  at  which  we  arrived.  ^  It 
will  be  seen  that  the  experimental  results  obtained  during 
the  last  thirty  years  on  the  sparking  distances  in  air  at 
ordinary  pressures,  when  no  coronas  are  formed,  could  have 
been  predicted  with  considerable  accuracy  from  the  above 
results. 
Principal  Carey  Foster  also  suggested  the  formula 
V  =  2-13  +  30-6  tf, 
for  the   spa 
riiino- 
potentials  between  2*6  cm.  knobs.     It  is 
interesting  to  notice  that  Bailie  and  many  other  experi- 
menters subsequently  suggested  linear  formulas  for  the 
relation  between  Y  and  x. 
In  reply  to  a  question  by  Professor  Poynting,  I  have 
worked  out  the  values  of  Rmax.  for  the  case  of  Heydweiller 
and  Algermissen's  tests  on  the  assumption  that  the  ordinary 
electrostatic  equations  hold,  without  modification,  at  the 
instant  of  breaking  down.     In  this  case  we  have 
EU  =  (V/«)/, 
where/'  can  be  found  by  formula  (20)  given  above.     The 
values  of  Rmax  are  those  found  in  Tables  VI.  and  VII. 
Heydweiller's  Test. 
i 
Maximum  value. 
:                                        i 
Minimum  value. 
Mean  value. 
Emax      
37-5+0-7 
37-5-0-6 
37-5 
R 'max. 
38-35+0-85 
3835-0-55 
38-35 
Algermissen's  Test. 
Maximum  value. 
Minimum  value. 
Mean  value. 
Rma 
37+0-4 
37-0-5 
370 
R'max 
374+0-3 
37-4-0  3 
37-4 
The  last  result  is   so  remarkable  that  I  give  the  complete 
table. 
T  2 
