326 



Royal Society, 



of the absolute electrometer, and results have been obtained con- 

 firming the general character of those shown in the preceding Tables. 

 The absolute evaluations derived from these later series, must be 

 more accurate than those deduced above from the single series of 

 December 13, when the distance between the plates in the absolute 

 electrometer was only *2 of an inch. I therefore by permission add 

 the following Table of absolute determinations : — 



These results, as well as those shown in the preceding Tables, 

 demonstrate a much less rapid variation with distance, of the electro- 

 static force preceding a spark, at the greater than at the smaller 

 distances. It seems most probable that at still greater distances the 

 electrostatic force will be found to be sensibly constant, as it was 

 certainly expected to be at all distances. The limiting value to 

 which the results shown in the last Table seem to point must be 

 something not much less than 2800. This corresponds to a pressure 

 of 9600 grains weight per square foot. We may therefore conclude 

 that the ordinary atmospheric pressure of 14,798,000 grains per 

 square foot, is electrically relieved by the subtraction of 9600 on two 

 very slightly convex metallic surfaces, at a distance of ^_th of an 

 inch or more, before the air between them is cracked and a spark 

 passes. By taking into account the result of my preceding com- 

 munication to the Royal Society, we may also conclude that a 

 Daniell's battery of 5510 elements can produce a spark between two 

 slightly convex metallic surfaces at ^th of an inch asunder in 

 ordinary atmospheric air. 



