www.physics.org... physics.org physics.org www.physics.org... two-thirds the size of the Moon physics.org physics.org http://www.physics.org/article-questions.asp?id=64 responsible for the earth’s magnetic field rooted deep inside the planet they generate electric currents www.physics.org... physics.org http://www.physics.org/article-questions.asp?id=64 www.physics.org... two-thirds the size of the Moon Earth’s magnetic sphere they generate electric currents http://www.physics.org/article-questions.asp?id=64