Dielectric  Strength  of  Air.  271 
An  examination  of  the  experimental  results  obtained  and 
the  methods  of  calculating  Rmax.  from  them  shows  that  the 
second,  third,  and  fourth  of  the  above  tests  are  the  only  really 
satisfactory  ones.  The  mean  of  the  results  obtained  in  these 
three  tests  is  38' 2  kilovolts  per  centimetre,  and  this  agrees 
closely  with  the  number  we  obtained  from  the  direct  pressure 
experiments. 
16.    Conclusion. 
We  conclude  therefore  that  the  dielectric  strength  of  the 
air  at  ordinary  atmospheric  pressures  lies  between  38  and 
39  kilovolts  per  centimetre,  which  is  about  30  per  cent,  greater 
than  the  value  ordinarily  given.  J.  J.  Thomson  *  gives  the 
value  as  approximately  30  kilovolts  per  centimetre  and 
M.  0 'Gorman  |  as  27  kilovolts  per  centimetre. 
The  confidence  of  electricians  who  are  responsible  for  the 
working  of  high-pressure  net-works  for  the  distribution  of 
electric  power  J  on  the  working  of  their  spark-gap  safety- 
valves  at  the  moment  the  pressure  attains  a  definite  value, 
and  the  extensive  use  they  make  of  micrometer  spark-gaps  § 
for  measuring  high  voltages,  prove  that  under  ordinary 
working  conditions  they  find  that  the  dielectric  strength  of 
air  is  approximately  constant.  In  ordinary  work  we  may 
take  its  value  as  38  kilovolts  per  centimetre. 
Appendix  I. 
The  Disruptive  Voltages  for  Large  Spherical  Electrodes. 
Table  XIII.  gives  the  disruptive  pressures  in  kilovolts 
between  equal  spherical  electrodes  when  their  radii  are  1,  10, 
100,  and  1000  cms.  respectively.  The  dielectric  strength  of 
air  has  been  taken  as  38  kilovolts  per  centimetre,  and  V  and 
V/  are  calculated  by  the  formulas 
V  =  0-8  +  Rmax.(.i'//)   and  Y'=V/<y% 
respectively.  V  therefore  gives  the  effective  value  of  the 
disruptive  voltage  when  the  pressure  is  alternating  and  sine- 
shaped. 
*  J.  J.  Thomson, '  Electricity  and  Magnetism,'  p.  59  (1901). 
t  M.  OGorman,  "Insulation  on  Cables,"  Journal  of  the  Inst,  of  Elect. 
Eng.  vol.  xxx.  p.  666  (1901). 
X  Dusaugey,  Soc.  Int.  Meet,  Bull.  5,  pp.  109-132.  "Methode  de 
Protection  contre  les  Surtensions  actuellement  employee  dans  les  Reseaux 
de  Transport  d'Energie,5'  Feb.  1905. 
§  P.  H.  Thomas,  Amer.  Inst.  Electr.  Engin.  Proc.  xxiv.  pp.  705-742, 
"  An  Experimental  Study  of  the  Tlise  of  Potential  on  Commercial  Trans- 
mission Lines  due  to  Static  Disturbances  caused  by  Switching,  Grounding, 
etc.",  July,  1905. 
