242 Mr. P. J. Edmunds on the 



wire to the point where the force is 30 kilovolts per centi- 

 metre, then 



4A . 



30 = 



As + a 



The average force over this distance s is approximately 

 Xx + 30 



Xi + 30 =3Q j 1-35 



9, .« 



or Xx-30 _T35 



2 " s ' 



30 = — -. — : — = -. — — ; -^ij 



4^_X 1 -30 



a -~~ 30 ' 

 /. (X!-30) 8 _ 1-35x4 

 60 "" a ' 

 18 



X 1 = 30 + 



a log — . -, o N 



.-. Y.-A^— ^-(l.^ 



This formula gives the sparking potential in kilovolts for air 

 at atmospheric pressure. / ^ 



Since Vi is a function of lap. - 1, for other pressures 



where P is the pressure in atmospheres. 



The formula is obtained on the supposition that -% and aP 



are both small. For the smaller wires it is found that the 

 calculated values lie between those found experimentally for 

 positive and negative discharges. The agreement for the 

 larger points is not very good, though the formula gives 

 results of the correct order of magnitude for quite large 

 points. 



A table has been made up showing the calculated and 



