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Electrically charged iron and aluminum particles having diameters of a few microns have been contained in a confined region of space by means of alternating and static electric fields. The theory is essentially that of alternating gradient focusing; here the motion is governed by Mathieu's equation. Under certain circumstances when many particles are confined the three dimensional focusing force and the Coulomb repulsion results in a ``crystaline'' array which can be ``melted'' and reformed.
Wuerker et al. (Sun,) studied this question.