AND MODERN PHYSICS. 183 



point is proportional to the electric force at that 

 point 



Thus, (i) when electric force acts on a conductor, it 

 produces a current, the current being by Ohm's law 

 proportional to the force: (ii) when it acts on an 

 insulator it produces electric displacement, and the 

 displacement is proportional to the force ; while (iii) 

 there is magnetic force in the neighbourhood of the 

 current, and the work done in carrying a magnetic 

 polo round any complete circuit linked with the 

 current is proportional to the current. The first two 

 of these principles give us two sets of equations con- 

 necting together the electric force and the current 

 in a conductor or the displacement in a dielectric 

 respectively; the third connects the magnetic force 

 and the current. 



Now let us go back to the variable period when 

 the current is flowing in the wires ; and to make ideas 

 precise, lot the two conductors be two equal large flat 

 plates placed with their faces parallel, and at some 

 small distance apart. In this case, when the plates 

 are charged, and the "current has ceased, the electric 

 displacement and the force are confined almost entirely 

 to the space between the plates. During the variable 

 period the total flow at any instant across each section 

 of the wire is the same, but in the ordinary sense of 

 the word there is no flow of electricity across the 

 insulating medium between the plates. In this space, 

 however, the electric displacement is continuously 

 changing, rising from zero initially to its final steady 

 value when the current ceases. It is a fundamental 

 part of Maxwell's theory that this variation of electric 



