194 ELECTROMETRY. 



depends on the nature of the dielectric which separates them, and, 

 if this is a gas, on the pressure and temperature of the gas, etc. 



In a gas under the ordinary pressure, the disruptive discharge 

 is produced under the form of a spark or brush, the latter taking 

 place in conductors which present points. We may pass from one 

 form to the other by merely modifying the capacity of the conductor. 

 This is readily confirmed by Holtz's machine, which gives brushes 

 in ordinary conditions, and sparks when the capacity of the con- 

 ductors is increased by connecting them with Leyden jars. Experi- 

 ment shows that the distance at which the discharge is produced, 

 or the striking distance, corresponds in both cases to the same 

 difference of potential. At each spark this difference suddenly 

 sinks almost to zero, as if the spark set up a momentary connection 

 between the conductors. With the brush the difference of potential 

 remains nearly constant. The brush, representing discharges of 

 small quantities of electricity which succeed each other with great 

 rapidity, forms thus a kind of valve. 



In rarefied gases the discharge is produced in the form of lumi- 

 nosities, which are very different according to the conditions. For 

 very low pressures in particular it forms bands which are alternately 

 bright and dark. Even with continuous sources, the luminosities 

 are due to a very rapid succession of discharges. When the vacuum 

 is made as complete as possible, no discharge takes place. 



822. STRIKING DISTANCE. Numerous researches have been 

 made to determine the relation which exists between the striking 

 distance and the difference of potential. From the experiments 

 of Harris* and of Riess,f the striking distance is nearly proportional 

 to this difference. If the law was general, we should conclude for 

 the case of two parallel plates that the production of electricity 

 almost represents the same value of the electrical density, and 

 therefore of the electrical force and the electrostatic pressure, or, 

 as in Maxwell's views (107), to the same condition or the same 

 specific energy of the interposed medium. But the more recent 

 experiments have not confirmed the generality of this law. 



Sir W. Thomson measured with an absolute electrometer the 

 difference of potential corresponding to the production of a spark 

 between two parallel plates, one in connection with the source 

 and the other in connection with the earth. In order to prevent 

 the spark from striking on the edges, the latter is very slightly 



* HARRIS. Phil. Trans. Roy. Soc. t p. 225. 1834. 

 t RIESS. Reibungselectricitdt, Vol. I., p. 377. 



