For example, consider the magnetic field of a loop of radius R carrying a current I . For a point a distance x along the center line of the loop, the magnetic field vector at that point is: B ( x ^ ^ ) = μ μ 0 I R 2 2 ( x 2 + R 2 ) 3 / 2 x ^ ^ , where x ^ ^ is the unit vector of along the center-line of the loop (and the loop is taken to be centered at the origin). Loops such as the one described appear in devices like the Helmholtz coil, the solenoid, and the Magsail spacecraft propulsion system. Calculation of the magnetic field at points off the center line requires more complex mathematics involving elliptic integrals that require numerical solution or approximations.