318 



Royal Society : — 



These numbers demonstrate an unexpected and a very remarkable 

 result, — that greater electromotive force per unit length of air is 

 required to produce a spark at short distances than at long. When 

 it is considered that the absolute electrification of each of the op- 

 posed surfaces* depends simply on the electromotive force per unit 

 length of the space between them, or, which is the same thing, the 

 resultant electrostatic force in the air occupying that space, it is 

 difficult even to conjecture an explanation. Without attempting to 

 explain it, we are forced to recognize the fact that a thin stratum of 

 air is stronger than a thick one against the same disruptive tension 

 in the air, according to Faraday's view of its condition as trans- 

 mitting electric force, or against the same lifting electric pressure 

 from its bounding surfaces, according to the views of the 18th cen- 

 tury school, as represented by Poisson. The same conclusion is 

 established by a series of experiments with the previously-described 

 portable torsion electrometer substituted for the absolute electro- 

 meter, leading to results shown in the following Table. 



Table II. — January 17, 1860. Measurements by portable torsion 

 electrometer of electromotive forces producing sparks across a 

 stratum of air of different thicknesses. 



The series of experiments here tabulated stops at the distance 

 18 thousandths of an inch, because it was found that the force in the 



* See § 4 below. 



