The expansion of an arc of helical form is governed by both electromag
netic and aerodynamic forces, themselves functions of the arc current
and electric field strength through complex interactions. In attemptin
g to describe the arc's expansion the product of these two quantities,
the power input, can be usefully exploited. As the arc's helical leng
th increases, the arc voltage, leading to current limitation and inter
ruption. Experimental results showing the evolution of electric field
strength with current are also presented.