(1) The chemicals in a battery. When connected to an electrical circuit, the chemicals in the electrolyte react to produce an excess of electrons at the negative terminal and a deficiency of electrons at the positive terminal. (2) A substance that dissolves in water to form particles with positive and negative electrical charges (ions), making the water a conductor of electric current. (3) A substance that transfers the ions being emitted during corrosion from the anode to the cathode. When referring to underground storage tank corrosion, the substance is typically moist soil.