2GG 
Mr.  A.  Bussell 
the 
error  in  determining  the  ratio  of  the  maximum  to  the  effective 
potential- difference.  An  error  is  also  due  to  the  moisture 
in  the  air.  This,  however,  was  found  to  be  small.  When 
the  electrodes  were  immersed  in  "  live"  steam  at  atmospheric 
pressure,  the  effect  of  the  steam  was  to  increase  the  apparent 
dielectric  strength  of  the  air,  a  greater  voltage  being  required 
to  produce  the  disruptive  discharge.  As  pressures  up  to 
160  effective  kilovolts  were  employed,  the  sparking-distances 
were  large  and  could  be  measured  with  great  accuracy.  We 
should  expect  that,  with  these  high  voltages,  our  formulae 
would  apply  with  considerable  accuracy,  as  the  disturbing 
effect  of  the  cathode  glow  would  be  small  and  the  field 
would  be  approximately  symmetrical. 
In  the  following  table  the  results  of  tests  when  the 
electrodes  were  spheres  2  inches  in  diameter  are  analysed. 
The  column  headed  Rmax.  gives  the  values  of  the  dielectric 
strength  in  kilovolts  per  centimetre,  calculated  by  the  formula 
Rmax  ={  (1-42  V-  0-8)  /*}/. 
Table  IX. 
C.  P.  Steinmetz.     2  inch  spheres  {a  =  2*54  cms.). 
=  125.     E/V  =  1*42,  where  E  is  the  maximum  and 
Y  the  effective  value  of  the  alternating  voltage. 
JSTo.  of 
Experiment. 
1 
2 
3 
4 
5 
6 
7 
8 
9 
10 
11 
12 
1 
X. 
0-318 
xja. 
/(calc). 
V(obs.). 
0-125 
1-04 
8-95 
0-635 
0-25 
1-08 
15-9 
1-25 
0-49 
1-17 
26  7 
2-74 
"-•08 
1-39 
51-0 
3-69 
1-45 
1-54 
65-2 
4-29 
1-69 
1-63 
70-8 
5-72 
2-25 
1-88 
83-8 
7-62 
3-00 
2-21 
94-0 
8-74 
3-44 
2-42 
1020 
100 
3-95 
2-66 
101-5 
12-9 
5-08 
3-19 
108-0        i 
142 
5-60 
3-44 
114-5 
K,. 
(calc.). 
390 
37-1 
34-7 
36-2 
38-3 
37-9 
38-9 
36-7 
39-9 
38-0 
37-7 
39-1 
The  mean  value  of  Emax.  obtained  from  the  figures  in  the 
last  column  is  37*8.  Considerable  importance  is  attached  to 
this  test  as  the  numbers  actually  observed  are  given. 
The  curve  in  fig.  4  gives  the  relation  between  V  and  x  on 
the  supposition  that  Bmax.  is  38.  Steinmetz's  experimental 
results  are  plotted  in  this  figure  for  purposes  of  comparison. 
