I am attempting to build a relay from constituent parts of Meccano Elektrikit. There are instructions for such a device in the set's manual, but the finished item (and all modifications I have tried) has a problem. If I arrange the parts so that the coils have enough force to pull in the armature, then the armature is not always released when the current is switched off. It sticks. If I try to manually release it at this stage, I can feel the 'stickiness'. If I then immediately manually move the armature back to its closed position, the stickiness has disappeared so there doesn't appear to be any residual magnetism in the core at that stage. If I don't manually release the armature, it will stay stuck for several minutes.
The relay assembly has two coils connected in series (I'm wanting the relay to switch over two separate circuits) and the fixed end of the cores are bolted to a steel base plate.
I have tried arranging the circuit with the coils' fields in opposite directions and in the same direction but have observed the problem in both cases. I have also tried many variations on gap spacing, the mass of the armature and the springiness of its mounting.