A watch battery, button cell, silver button cell, or coin cell is a small form-factor battery esigned for use in wrist watches, pocket calculators, hearing aids, and similar compact portable electronics products. They are compact and have long life. Examples are mercury oxide and silver oxide cells. A mercury oxide cell has a mercury oxide cathode and a zinc anode. Watch batteries are usually a single cell with a nominal voltage between 1.5 and 3 V. Common anode materials are zinc or lithium, common cathode materials are manganese dioxide, silver oxide, carbon monofluoride or copper oxide. The cylindrical sides of these types of batteries is part of the positive (+) terminal.