A solenoid is a coil wound many times on a cylinder of length greater than its diameter. Solenoids are mainly used as electromagnets, because a magnetic field is formed when current flows through a solenoid. The solenoid described in secondary school and university textbooks is a single-layered solenoid. Further, textbooks and papers focusing on education primarily describe the magnetic fields caused by infinite solenoids. However, most solenoids used in laboratory or daily life are finite solenoids with multiple layers to achieve a strong magnetic field. Examples include solenoid valves, Faraday rotators, and modulators.