In my laboratory class we were doing an experiment with grass seeds and iron filings to visualize the electric and magnetic field lines. So, we were discussing why they appear, because we know that the fields are continuous (ie, they don't have holes or empty spaces) and however the field lines appear. We believe that is because of the size of the grass seeds and filings, and if we could use really small particles, we would visualize the "continuity" of the field. Can I have more opinions?
I have read about these fields and authors discuss the math behind the field, but not this kind of situation.