(a) If two terminals of an active circuit are connected to two points of
a thin metallic plate the current spreads over or occupies practically a
considerable area of such plate, and this portion of the current is a
current sheet.

The general contour of the current sheet can be laid out in lines of
flux. Such lines resemble lines of force. Like the latter, they are
purely an assumption, as the current is not in any sense composed  of
lines.

(b) A condition of current theoretically brought about by the Ampérian
currents in a magnet. Each molecule having its own current, the
contiguous portions of the molecules counteract each other and give a
resultant zero current. All that remains is the outer sheet of electric
current that surrounds the whole.