Pressure on the Length of Disruptive Discharge in Air. 189 



atmosphere Harris's law approximately holds good. No va- 

 riation from it indicating any other law is observed. 



(2) No law can be said to be more than approximately true; 

 for when the density has almost reached the discharging limit, 

 any slight accidental circumstance, such as the presence of a 

 grain of dust, a little burning of the point by the last dis- 

 charge, &c, will cause the discharge to take place. Professor 

 Clerk Maxwell has compared the experiment to the splitting 

 of a piece of wood by a wedge. It is possible to determine 

 the average pressure on the wedge which will split the wood ; 

 but in any particular experiment it is impossible to say that 

 the wood will split exactly at that pressure. 



(3) When the pressure is diminished below 11 inches, the 

 product in column VII. rapidly diminishes. This shows that 

 at low pressures the spark produced by a given electromotive 

 force is much shorter than is required by Harris's law, or that 

 the electromotive force required to produce a spark of given 

 length is at low pressures greater than that required by Harris's 

 law. This agrees with what Mr. De La Rue has told me, 

 namely that he finds that at all pressures, however low, the 

 discharge is disruptive, and none of it passes by conduction. 

 If any portion could at low pressures pass by conduction, we 

 might expect that a smaller and not a greater electromotive 

 force would be required than that calculated by Harris's law 

 from experiments at high pressures. 



It is also not inconsistent with the result of Sir William 

 Thomson's historical experiments (mentioned above) "On the 

 Electromotive Force required to produce a Spark." For he 

 writes*, " Greater electromotive force per unit length of air 

 is required to produce a spark at short distances than at long." 

 For the words in italics I substitute "at low pressures than at 

 high." We may then both write "with a low air resistance 

 than with a high one," or " with few air particles between the 

 points than with many." Sir William Thomson says of his 

 result, "it is difficult even to conjecture an explanation;" I 

 can only say the same of mine. 



I cannot say exactly at what pressure M. Knochenhauer's 

 experiments show a change in the law — as he evidently con- 

 siders the change to be due partly at least to experimental 

 errors, and introduces corrections, some of which are appa- 

 rently suggested by peculiarities in his apparatus, while others 

 are intended to adjust the experimental results to the supposed 

 law. As far as I can see, the change was first beyond the 

 reach of his corrections when the barometer fell to about 2 

 inches. 



* Papers on Electrostatics and Magnetism, § 323, p. 248. 



