A method for drilling a well includes applying a current across a drill collar that surrounds an internal magnetometer. The current travels across the drill collar and along one or more other current paths to return to the drill collar. The method also includes using the magnetometer to measure magnetic fields due to the current being applied across the drill collar and the current across one or more other current paths while the drill collar is disposed at a first angle of rotation and rotating the drill collar to a second angle of rotation. The method further includes measuring the magnetic fields using the internal magnetometer while the drill collar is disposed at the second angle of rotation and determining a relationship between the current across the drill collar and a portion of the magnetic fields due to the current across the drill collar measurable by the internal magnetometer.