262 
Mr.  A.  Russell  on  the 
distance  between  them  was  l/8th  of  a  centimetre.  Taking  the 
E.M.F.  of  a  Daniell  cell  as  1*07  volts,  this  makes  the 
dielectric  strength  Rma5.  of  air  to  be  40*8  kilovolts  per 
centimetre :  a  result  which  is  only  about  6  per  cent,  higher 
than  the  number  we  give  as  the  average  value  of  Rmax- 
(ii.)  A.  Heydweiller.     5  cm.  spheres  (2*5  cm.  radius). 
In  the  valuable  paper  by  A.  Heydweiller,  published  in  the 
Annalen  der  Physik  und  Chemie,  vol.  xlviii.  p.  785  (1893), 
there  are  many  tables  of  sparking- distances  given  both 
between  equal  and  unequal  electrodes.  We  consider  merely 
the  last  table  he  gives,  and  we  choose  the  5  centimetre 
spheres  as  being  likely  to  give  the  most  accurate  results. 
The  height  of  the  barometer  was  74' 5  cms.,  and  the  tempera- 
ture 18°  C.   during  the  test.      The    columns   headed  x  and 
V  are  taken  from  Heydweiller's  paper,  fx  is  calculated  by  the 
formulae  given  above,  and  Rmax.  is  found  by 
Bnu«.={lV-0-8)M/1. 
We  have  assumed   that  the  potentials  of  the  spheres  are 
V  and  0  at  the  instant  of  the  discharge.  The  results  seem 
to  indicate  that  this  was  not  the  case  when  the  electrodes  were 
at  their  greatest  distances  apart. 
Table  VI. 
Heydweiller's  test  with  5  cm.  spheres  (a  =  2*5). 
;c=distance  apart  in  cms.         Y=disruptive  pressure  in  kilovolts 
X. 
x  a. 
/,  (calc). 
V  (observed). 
Umax.  (calc).      \ 
0-5 
ol 
1-068 
1S-36 
37-5 
0-6 
0-24 
1-081 
21-60 
37-5 
07 
0-28 
1-102 
24-54 
37-3 
08 
0-32 
1116 
27-33 
37-0 
0-9 
0-36 
1-132 
30-09 
36-9 
1-0 
0-40 
1-150 
32-85 
36-9 
1-1 
0-44 
1-169 
35-58 
37-0 
1-2 
0-48 
1-188 
38-31 
370 
1-3 
0-52 
T209 
41-01 
37-4 
1-4 
0-56 
1-231 
43-68 
37-7 
1-5 
060 
1-253 
46-23 
37-9 
1-6 
0-64 
1-277 
48-66 
38-2 
The  mean  value  of  the  numbers  in  the  last  column  is  37*5, 
and  none  of  them  differ  from  the  mean  value  by  as  much  as 
2  per  cent.  Hence  this  experiment  gives  37*5  kilovolts  per 
centimetre  as  the  dielectric  strength  of  air. 
