Effect of Pressure on Disruptive Discharge, 185 



height above ground, The ratio 



M 



expresses the ratio of the strength of the sinal wave currents 

 in the wire containing the electromotive force E sin mt to the 

 corresponding induced currents in the parallel wire, or, rather, 

 the minimum value of that ratio for rapid reversals. For if 

 Ri, L x belong to the primary circuit, R 2 , L 2 to the secondary, 

 then 



r i = Ey/Rjj + LoW 



s/ { BaRa - m 2 ( L i L 2 - M 2 ) } 2 + m\ R X L 2 + RA) 2 

 and 



Mm 



As m increases, this approximates to 



T 2 M 



i\ ~ U 



equal to about ^ if there are no electromagnets in the second- 

 ary circuit, otherwise much less, It is here assumed that 

 « = 25 for the iron wire. 



XXV. On the Effect of Variation of Pressure on the Length of 

 Disruptive Discharge in Air. By J. E. H. Gordon, B.A., 

 Assistant General Secretary of the British Association* . 



[Plate in.] 

 History. 



IN 1834 Mr. Snow Harris stated f that, other things being 

 equal, the length of the spark which an electric machine 

 or Ley den jar will give in air varies in the simple inverse 

 ratio of the pressure. He, however, gives no tables or figures 

 in support of his law. 



Sir William Thomson J has determined, by means of an ab- 

 solute electrometer, the difference of potentials corresponding 



* Communicated by the Author, having been read before Section A of 

 the British Association, Dublin, 1878. 

 t Philosophical Transactions. 

 t Proc. Roy. Soc. 18G0, Papers on Electrostatics, p. 247, 



