ELECTRIC CHARGE. THE CONDENSER. 



177 



96. The spark gauge. The electromotive force required to 

 produce a spark between two polished metal spheres of given size 

 in air varies in a definite manner with the length of the air gap. 

 If the electromotive forces required for different distances be once 

 determined by observation, then any electromotive force may be 

 determined by measuring its sparking distance between the pair 

 of spheres. An arrangement for measuring electromotive force 

 in this way is called a spark gauge or a spark micrometer. The 

 spark gauge is adapted only to high electromotive forces, and the 

 results obtained by it are subject to large errors. 



TABLE. 

 Sparking distances in air at 18 C. and 745 mm.* pressure. 



s = length of air gap in centimeters. 

 r = radius of spheres in centimeters. 



97. Electric flux. The product of the intensity of an electric 

 field and an area at right angles to the direction of the field is 

 called the electric flux across the area , that is, we may write 



in which 



3> =fa (68) 



is the electric flux across a square centimeters of 



* Heydweiller, Wied. Ann. 48, p. 235, 1893. 

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