Electricity is the flow of charged particles through a material, such as a wire — a process that resembles a river of water molecules flowing through a canyon. But are the charged particles positive or negative? Benjamin Franklin had no way of knowing, so he guessed they were positive. We now know that when electricity flows through metals, the particles are negatively charged. They are electrons.
Scientists at Cornell University have made the startling discovery that they can change the sign of the current-carrying particles in one type of ceramic, based on ruthenium oxide, from negative (electrons) to positive (missing electrons or “holes”) by simply stretching the material. This stretching technique may be useful for creating new classes of materials that have carefully tuned or “designer” electronic properties.