Evolution of the magnetic polarities in the 2D flux cancellation experiment. The black lines show the horizontal position of each source, given by Eq. (25). The blue lines show the horizontal position of the photospheric polarities, that is, the location along z = 0 where B_z reaches its maximum value. The results here are from the simulation with the Gudiksen-11 resistivity model, but nearly identical results are obtained with the other two resistivity models.