The modern development of batteries started with the Voltaic pile, invented by the Italian physicist Alessandro Volta in 1800. A battery is a device that converts chemical energy directly to electrical energy. It consists of one or more voltaic cells. Each voltaic cell consists of two half cells connected in series by a conductive electrolyte. One half-cell is the negative electrode (the anode) and the other is the positive electrode (the cathode). In the redox reaction that powers the battery, reduction occurs in the cathode, while oxidation occurs in the anode. The electrodes do not touch each other but are electrically connected by the electrolyte, which can be either solid or liquid. In many cells, the materials are enclosed in a container, and a separator, which is porous to the electrolyte, which prevents the electrodes from coming into contact.