274: 
Mr.  A.  Russell  on  the 
so  that  the  frequency  is  nearly  160,  and  the  potentials  of  the 
electrodes  be  equal  and  opposite  at  every  instant,  we  have 
A  =  o>KV  =  1000  x  0-889  x  10  x  90  x  103/(9  x  105  x  106) 
=  0-000889  ampere. 
If  the  potentials  of  the  electrodes  be  not  equal  and  opposite 
at  every  instant,  the  difference  between  the  effective  values 
of  the  capacity  currents  to  the  electrodes  equals  the  effective 
value  of  the  current  in  the  earth  connexion.  Our  formulas 
are  not  applicable  when  the  electrodes  are  surrounded  with 
coronse.  In  this  case  the  capacity  between  them  is  con- 
siderably increased. 
The  author  is  indebted  to  Mr.  Arthur  Berry  for  suggesting 
the  use  of  Clausen's  theorem  as  an  aid  in  calculating  the 
capacity  coefficients  of  two  equal  spheres. 
Postscript. 
I  have  received  from  Principal  G.  Carey  Foster  the  results 
of  experiments  made  in  his  laboratory  in  1876  on  the  sparking 
distances  between  2' 6  cm.  brass  knobs.  As  these  results  are 
of  considerable  interest  I  have  obtained  his  permission  to 
publish  them. 
G.  Carey  Foster. — 2*6  cm.  spheres  (a=l'3). 
1 
X. 
xja. 
f- 
V  fobs.). 
005 
0-0385 
1-013 
3-09 
46-4 
o-i 
0-0769 
1-026 
504 
435 
0-2 
0-1538 
1-051 
8-43 
40-0 
03 
02307 
1-077 
11-46 
38-3 
0-4 
03076 
1-105 
14-61 
38-1 
0-5 
0-3846 
1-131 
17-49 
37-7 
0-6 
0-4614 
1-159 
20-43 
37-9 
07 
0-5383 
1-185 
2337 
38-2 
0-8 
0-6152 
1-213 
2625 
38-5 
0-9 
0-6921 
1-242 
29-13 
39-0 
A  l  home-made '  absolute  electrometer  was  used  to  measure 
the  voltage.  The  attracted  disk  was  hung  from  an  ordinary 
balance  and  the  attraction  weighed  directly.  Up  to  30  kilo- 
volts  it  gave  trustworthy  readings. 
Neglecting  the  first  value  of  Rmax.  as  the  formula  given  in 
the  paper  is  only  roughly  applicable  when  the  distances  are 
less  than  one  millimetre,  we  find  that  the  mean  value  of 
Emax.  is  39.     If  we  neglect  the  first  two  readings  the  mean 
