Some of my groups get very close to building the magnet, but none are working at the end of the time. I ask the class if they are ready to see a model of the magnet and the capability it will have once working. They are all eager to see. I call them over and model the wrapping of the wire and then how I connect it to make the circuit. When wrapping the wire I discuss with them what we learned about south and north poles.
When I finish building the magnet, I then demonstrate how it will pick small washers. This is amazing to the class and they are eager to get back their stations to build.
By modeling after exploring students are able to see how where they were close to having it working. They can also determine what was not going to work and can evaluate their thinking so far.