| Astatic means that the instrument is constructed in a way that minimizes the effect of the earth's magnetic field. Electrical current being measured passes through coils and produces a magnetic field which causes the magnetized needle suspended above the card to rotate. Not visible is a second needle, rigidly attached to the one above, which hangs below the card in a region where the magnetic field is the opposite of the field above. That needle has the opposite polarity from the one above, so that the earth's field acting on both needles has negligible effect. |  |