ELECTROMAGNETIC THEORY OF LINES AND SHIELDS 535 



current in a circular turn of wire is surrounded by a field of this type; 

 in this case, the lines of electromotive intensity form a coaxial system 

 of circles. 



TWO-DIMENSIONAL FlELDS 



By definition, two-dimensional fields are constant in some one 

 direction. If we take the z-axis of our reference system in this direc- 

 tion, all the partial derivatives with respect to z vanish, 2 disappears 

 from our equations and we can confine our attention to any plane 

 normal to the z-axis. 



Once more the set of six electromagnetic equations breaks up into 

 two independent subsets. One of these is - 



1 dH, 1 dH. 



(g -f- icoe)p dip ' g + -icoe dp 



1 



P 



d(pE^) ^ dE, 



(4) 



= — ioinHf 



dp d(p 



The calculation of what is commonly known as "electrostatic" cross- 

 talk between pairs of parallel wires is based upon these equations. 

 For this reason we shall name the field defined by (4) the electric 

 field. 



Similarly, the remaining three equations define the magnetic field : 



. , _ _ J_ aE, zr - _ X ^^ 



loifxp ocp iwp. op 



dp dtp 



(5) 



{g + iwe)E^ 



and are useful in the theory of what is generally known as "electro- 

 magnetic" crosstalk. 



The distinction between electric and magnetic fields is purely prag- 

 matic and is based upon a necessary and valid engineering separation 

 of general electromagnetic interference into two component parts. 

 In some respects the firmly entrenched terms "electrostatic crosstalk" 

 and "electromagnetic crosstalk" are unfortunate; it would be hopeless, 

 however, to try a change of terminology at this late stage of engineering 

 development. 



Further consideration of two-dimensional fields will be deferred 

 until the problem of shielding is taken up later in this paper (page 

 567). 



2 In passing from the original set (1) we reversed the sign of Ep in order to make 

 the set of equations symmetrical. The positive Ep is now measured toward the axis. 



^ In these equations, the sign of H^ was reversed so that the magnetomotive 

 intensity is now positive when it points clockwise. With this convention, the 

 flow of energy is away from the axis when both H^ and Et are positive. 



