http://en.wikipedia.org/wiki/Electric_arc
"An electric arc has a non-linear relationship between current and voltage. Once the arc is established (either by progression from a glow discharge or by momentarily touching the electrodes then separating them), increased current results in a lower voltage between the arc terminals. This negative impedance effect requires that some positive form of impedance to be placed in the circuit, if it is desired to maintain a stable arc. This property is the reason uncontrolled electrical arcs in apparatus become so destructive, since once initiated an arc will draw more and more current from a fixed-voltage supply until the apparatus is destroyed."
'ya learn somethin' new every day.