384 



RECENT PEOGRESS IN PHYSICS. 



micrometer) has no influence upon the brightness of the electrical 

 spark if the extent of the coating remain the same. The size of the 

 plate, however, has considerable influence. 



With unchanged distance of the constant light and of the striking 

 distance, and with the same thickness of glass, the surface of the 

 coating was varied, and each time the corresponding distance of the 

 spark from the rotating disk, at which the sectors just ceased to be 

 perceived, was observed. The results of such a series of observations 

 are given in the following table : 



The first vertical column contains the size of the surface of the coat- 

 ing, the second is the ratio of the first and second surfaces, and then 

 of the first and third. The third column gives the corresponding Y, 

 the fourth the square of this distance^, and the last the ratio of the 

 numbers of the first Y^ to the second, and of the first to the third. 



Comparing the second and fifth columns we have very plainly 





(1) 



that is, the superficial content F of the coating of the condenser is in 

 jiroportion to the square of the distance Y, the illumination of the 

 disk ab by the electrical spark remaining constant. 

 But the constant illumination of the disk is 



J = c Y2 



J denoting the intensity of the light of the spark, and c a constant 

 factor, or 



ys 



c. 



(2) 



Combining equations (1) and (2) we get 



J = -- F ; 



n 



or, the hriglitness of the electrical spark is proportional to the surface of 

 the coating. 



§ 92. lielation hetiveen the intensity of the electrical sparh and the 

 thickness of the condenser. — The surface of the coating remaining the 

 same, the thickness of the glass plate was changed ; and for each plate 

 the distance Y was observed at which the sectors just ceased to be 

 visible, the illumination by the lamp remaining constant. The fol- 

 lowing table contains a few of the results obtained. 



