542 Intelligence and Miscellaneous Articles. 



heat is evolved, as I have ascertained by introducing one of the 



jars into an air-thermometer suitably arranged. 



Therefore, besides the ordinary external discharge of the jar, 

 there is another in its interior, which I will call internal, to distin- 

 guish it from the former. It takes place along the sides of the 

 condenser where they are destitute of coating ; and it is appreciable 

 by the light and heat which accompany it. On measuring the in- 

 ternal discharge by the thermometric dilatations which it produces, 

 we arrive at the following conclusions : — 



1. The heat evolved by the internal discharge can be neglected 

 with feeble discharges ; beyond certain limits, however, it manifests 

 itself, and increases very rapidly with the discharges themselves ; 

 thus a principal means for augmenting this internal heat is to em- 

 ploy jars charged to a very high potential. 



2. The internal discharge increases sensibly if the external spark 

 is produced betwen two balls from 10 to 30 millim. in diameter ; 

 it is diminished, on the contrary, by almost the half, if the spark is 

 called forth between a point and one of the balls. It is the reverse 

 for the heat produced by the external exciting spark. 



3. The internal discharge increases, for a given charge, if the 

 internal coating of the jar be diminished; it diminishes if we 

 augment the coating until it meets the outer coating. From this 

 point it remains almost independent of the extent of the coating 

 within the limits in which I have operated. The reason of these 

 phenomena is complex : they depend in part on the variation under- 

 gone by the potential of the discharge with the extension of the 

 coating, and partly on the influence exercised by the different ex- 

 tents of the two coatings upon the number and size of the sparks. 



4. The internal discharge is the same with an ordinary as with 

 a spark-discharging jar. 



5. The internal discharge falls to zero when the resistance of the 

 external circuit is much increased. 



6. The internal discharge appears, cceteris paribus, a little stronger 

 with an internal coating of mercury. With this exception, the 

 jar behaves like an ordinary phial with a coating of tinfoil. 



The foregoing conclusions, deduced from the thermometric dila- 

 tations, are completely confirmed by the luminous phenomena ex- 

 hibited in the jars, since the brightness and size of the internal 

 sparks constantly correspond almost exactly with the extent of the 

 thermometric dilatations. 



7. The internal discharges depend, in my opinion, on this — that 

 each coating induces or excites in the insulating slip a zone charged 

 with opposite electricity to its own, the zones induced by the two 

 coatings being separated by another zone of glass in the natural 

 state. At the moment of the discharge a part of the electricity of 

 the coating and of the electrified zone neutralize each other with 

 production of sparks and heat ; hence the internal discharge. 



8. The existence of these electrified zones can be demonstrated 

 by the electric figures obtained on projecting upon a Franklin's 

 square of varnished glass, or, better, of ebonite, or on a charged 



