THE DIELECTRIC 101 



energy per charge on unit area in B will only be -^^ while 

 in A it is -j-' Or, to produce a given strain D in B, requires 

 only -r^ of the work needed to produce an equal strain D in A. 



The "electric modulus" of B, then, is only ^ that of air. 



We may put this more precisely if we consider the intensities in 

 the two condensers with equal charges. When the dielectric B is 

 solid, we cannot directly measure the intensity within it, but in 



order to account for the difference of potential -^ we must suppose 



that the intensity has -^ of the value which it has in A. If in 



the air condenser it is E = 4-7rD, then in the other it is E' = 47rD/K. 

 If we regard intensity as electric stress and D as electric strain, 

 then, using the analogy to ordinary elastic stress, we should define 

 the electric modulus as electric stress -~ electric strain. In air, 

 then, it is E^-D = 4?r. In the dielectric with constant K it is 



The account of Faraday's discovery is given in Series XI, vol. i, 

 of the Experimental Researches in Electricity, and to the 

 paper we refer the reader for details. It is well worth study not 

 only for the importance of the results but also as a splendid 

 example of Faraday's mode of thought and work. It will suffice 

 here to say that Faraday prepared two equal condensers, each 

 consisting of an outer hollow brass sphere on a stand and 

 an inner concentric brass sphere supported by a metal rod 

 passing up through a neck at the top of the outer sphere, and 

 fastened in position by a plug of shellac. The outer sphere was 

 made of two hemispheres like the Magdeburg hemispheres. The 

 rod terminated in a knob. Each condenser was thus virtually a 

 Leyden jar. Initially air was the dielectric in each. One of the 

 jars was charged and its knob was then touched by the carrier 

 ball of a Coulomb electrometer (see p. 63). The charge received 

 by the ball was measured and gave, as we should now express it, 

 the difference of potential between the coatings. Then the knobs 

 of the two jars were touched together and the charge was shared. 

 The carrier ball now showed that the charge was equally shared, 

 or the capacities of the jars were equal. The lower half of the 

 space between the spheres of one condenser, which we will call B, 

 was then filled with a hemispherical cup of shellac, while the 

 other condenser, A, still contained air only. A was charged, and 

 its potential in terms of the electrometer reading was found, after 

 certain corrections, to be 289. The charge was shared with B by 



