40-4 Dr. A. Macfarlane on the Disruptive 



one reading, and fell back after the passage of the spark almost 

 to its ultimate position. At the third distance, when the 

 charge was negative, hissing sparks, giving only very small 

 discharges (as indicated by the behaviour of the index), pre- 

 ceded the loud spark, which gave complete discharge ; but 

 when the charge was changed to positive, no hissing dis- 

 charges were observed preceding the loud discharge. At the 

 fourth distance hissing discharges preceded the loud dis- 

 charge in both cases ; but they were much more numerous in 

 the case of the negative than of the positive charge. 



The readings for the positive discharge and for the negative 

 discharge, when plotted, indicate each a curve similar to that 

 of diag. 6, but with these two differences — first, that in the 

 region of the single spark the negative curve is higher than 

 the positive, and, secondly, that it passes into the brushy 

 state at a less distance and then becomes lower. The read- 

 ings appear to indicate that the brushy sparks begin at the 

 same difference of potential for the two. Thus the preemi- 

 nence of the positive spark is due to the fact that it requires a 

 less difference of potential. 



This result agrees in one respect with, but differs in ano- 

 ther from, that obtained by Wiedemann and Btihlmann * under 

 somewhat similar circumstances. They used a ball 2*65 

 millims. in diameter, and instead of the plates a larger ball 

 of 13*8 millims. in diameter, which was not insulated like the 

 plate, but connected with the other conductor of the Holtz 

 machine. The pressure was also different, the greatest at 

 which the observations were taken being 55 millims. Their 

 result agrees in this, that the curve for the small ball nega- 

 tive lies above that for the small ball positive for distances 

 between 3 and 12 millims.; and differs in this, that their 

 curve for the small ball positive is convex towards the axis of 

 distances. 



Measurement of the Dielectric Strengths of different 



Substances. 



Prof. Chry stall has pointed out the two modes in which 



the dielectric strength of a medium may be measured : — 1st, 



by the value of the electrostatic force when the electricity is 



on the point of passing from one molecule to the next ; and 



2ndly, by the maximum value of that electric tension which, 



according to the theory of Faraday and Clerk Maxwell, exists 



in the dielectric when under induction. He uses the phrase 



dielectric strength to denote the latter ; as it is convenient to 



have a name for the other physical quantity, it may be called 



specific resistance. In the following table I have embodied 



* Pogg. Ann. cxlv. p. 372. 



t Encyc, Brit. " Electricity," p. 60. 



