any surface in the field such that the potential is the same at every
point of the surface, we have what is called an _equipotential surface._
The difference of potential between any two points is called an
electromotive force. The lines of force are necessarily perpendicular to
the surface. When the lines of force and the equipotential surfaces are
straight, parallel, and equidistant, we have a _uniform field._ The
intensity of the field is shown by the number of lines passing through
unit area, and the rate of variation of potential by the number of
equipotential surfaces cutting unit length of each line of force. Hence
the distances separating the equipotential surfaces are a measure of the
electromotive force present. Thus an electric field can be mapped or
plotted out so that its properties can be indicated graphically.

[Illustration: Fig. 1]

The air in an electric field is in a state of tension or strain; and
this strain increases along the lines of force with the electromotive
force producing it until a limit is reached, when a rent or split occurs
in the air along the line of least resistance--which is disruptive
discharge, or lightning.

[Illustration: Fig. 2]

Since the resistance which the air or any other dielectric opposes to
this breaking strain is thus limited, there must be a certain rate of
fall of potential per unit length which corresponds to this resistance.
It follows, therefore, that the number of equipotential surfaces per
unit length can represent this limit, or rather the stress which leads
to disruptive discharge. Hence we can represent this limit by a
length. We can produce disruptive discharge either by approaching the