394 Dr. A. Macfarlane on the Disruptive 



the electric tension p\ which figures so prominently in Clerk- 

 Max well's theory. For air at the atmospheric pressure we 

 found 



hence 

 and 



V= 66-94 </{s 2 + '205 s\; 

 R=66-94 N /{l+^}, 



On diagram 2 I have drawn the above curve for the electro- 

 static force, and also the curve deduced from Sir William 

 Thomson's results. The electrostatic force is not constant, as 

 the mathematical theory of electricity in equilibrium would 

 lead us to expect ; but it tends to become constant when the 

 length of the spark is increased. The fact that the ratio of the 



a to the - of the hydrogen-curve is equal to the corresponding 

 ct 



ratio for the air-curve, points to the gas as the cause of the 

 anomaly. By substituting a liquid for the gaseous dielectric, 

 I have been able to show that the anomaly depends on the 

 characteristics of a gas as compared with those of a liquid ; 

 and I shall adduce some experiments which were designed to 

 test whether one of these differential characteristics is, as 

 Clerk-Maxwell* suggested, a condensation of the gas on the 

 surface of the electrodes. The following table gives the nu- 

 merical values: — 



Length of 



spark, in 



centimetres, 



Difference of 

 potential in 

 O.GhS. units. 



Electrostatic 

 force, 



Electric 

 tension, 

 , KR 2 



s. 





s 



•025 



5-08 



203-1 



1322 



•05 



7-56 



151-2 



910 



•075 



970 



129-4 



666 



•1 



11-69 



' 116-9 



544 



•2 



19-05 



95-3 



361 



•3 



2606 



86-9 



300 



•4 



3293 



82-3 



270 



•5 



39-75 



79-5 



251 



•6 



46 52 



77-5 



239 



•7 



53-28 



76-1 



230 



•8 



60-02 



75-0 



224 



•9 



6676 



74-2 



219 



1-0 



73-48 



73-5 



215 



* ' Electricity and Magnetism,' vol. i. p. 56. 



