  We describe a simple physical method for trapping and transforming magnetic
colloid clusters on a current carrying wire. We use the wire's field as a mould
to form colloidal rings and helices on its surface. To transform the initially
amorphous, kinetically trapped, bulky clusters we induce a low frequency
magnetic modulation wave that spins around the wire axis, effectively
eliminates defects from the clusters and stretches them into slender rings and
helical structures. A qualitative theoretical model of the underlying resonant
transformations is developed and the practical potential of the wire as a
magnetic micro-assembler is discussed.
