Duffing's electromagnetic lock is a kind of electromagnet used to permanent attraction of magnet keeper and it fast releasing when additional coil will be energized.
A holding coil is constantly supplied with properly chosen current that saturates relatively thin iron core in the surroundings of releasing coil. If additional current flows there, a magnetic field will be significantly reduced. If the force of keeper attraction is comparable with the force of the spring, even small demagnetizing current will release the magnet keeper.
Given: Core and keeper material: iron magnetic permeability: nonlinear Holding coil total (n*I)h = 100'000 A·turns;
Releasing coil total current (n*I)r = 50'000 A·turns;
Force of stretching spring F = 250 N/m
Results: When releasing coil is not energized (I=0, edlock_1 case) then the force of keeper attraction is about 295 N/m. Otherwise (I>0, edloc_2 case) the attraction force decreases to 225 N and becomes less then stretching force of the spring, and magnet keeper will be unlocked. The force was calculated by means of "Integral calculator" tools.