If the charge on the rod were negative, the electric field would induce a dipole moment of reversed polarity, and the particle would still be attracted to regions of high electric field. (scientificamerican.com)
By generating high electric fields at the tip of the needle, Hughes can inject electrons into the molten polymer to create negatively charged ions. (newscientist.com)
Last month, however, the JILA group started up a new version of its experiment with higher electric fields in order to trap more ions simultaneously. (sciencemag.org)