Magnetic field lines form in concentric circles around a cylindrical current-carrying conductor, such as a length of wire. The direction of such a magnetic field can be determined by using the "right hand grip rule" (see figure at right). The strength of the magnetic field decreases with distance from the wire. (For an infinite length wire the strength is inversely proportional to the distance.) Bending a current-carrying wire into a loop concentrates the magnetic field inside the loop while weakening it outside. Bending a wire into multiple closely spaced loops to form a coil or "solenoid" enhances this effect. A device so
Field coil cost, would be silver as this has even lower resistivity. Silver has been used in rare cases. During World War II the Manhattan project to build the first atomic bomb used electromagnetic devices known as calutrons to enrich uranium. Thousands of tons of silver were borrowed from the U.S. Treasury reserves to build highly efficient low-resistance field coils for their magnets. Field coil A field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine, typically a rotating electrical machine such as a motor or generator. It consists of a coil of wire through which a