146 



ADVANCED ELECTRICITY AND MAGNETISM. 



17,500 volts per centimeter; and the introduction of a glass 

 plate 2 centimeters thick increases the stress on the remaining 

 2-centimeter layer of air to 30,000 volts per centimeter. 



This effect can be shown very beautifully by connecting two 

 metal plates to a high -voltage transformer, and adjusting the 

 plates to a distance such that the intervening air layer is barely 

 sufficient to sustain the voltage. Then if a glass plate be intro- 

 duced between the metal plates, the electrical stress in the 

 remaining air will be increased sufficiently to break the air down 

 at each reversal of the alternating voltage, as shown by the bluish 

 luminosity of the air layer. 



Magnetic analog. Figure 94 represents a narrow air gap 

 between the ends of two large magnetized iron rods. The 



iron 



iron 



Fig. 94. 



magnetic flux density <$ is the same in the air gap as in the iron 

 (see Art. 54). The permeability of the iron is, say, 1,000 and 

 the permeability of the air is unity (see Art. 57). Therefore the 



value of M (the magnetic field intensity) is 



IOOO 



in the iron and 



- in the air. That is, the magnetic field intensity is 1,000 



times as great in the low permeability air as in the high 

 permeability iron just as the electric field intensity is 6 times 

 as great in the air (having a low inductivity) as it is in the 

 glass (having a high inductivity) in Fig. 93. 



The concentration of the greater part of the voltage upon the 

 air layer in Fig. 93 is exactly analogous to the concentration of 



