  During eruptive solar flares and coronal mass ejections, a non-pot{\-}ential
magnetic arcade with much excess magnetic energy goes unstable and reconnects.
It produces a twisted erupting flux rope and leaves behind a sheared arcade of
hot coronal loops. We suggest that: the twist of the erupting flux rope can be
determined from conservation of magnetic flux and magnetic helicity and
equipartition of magnetic helicity. It depends on the geometry of the initial
pre-eruptive structure.
  Two cases are considered, in the first of which a flux rope is not present
initially but is created during the eruption by the reconnection. In the second
case, a flux rope is present under the arcade in the pre-eruptive state, and
the effect of the eruption and reconnection is to add an amount of magnetic
helicity that depends on the fluxes of the rope and arcade and the geometry.
