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Since neutral points occur where the two magnetic field strengths are equal and opposite, if one field is known then so is the other. For instance, if the current in a vertical wire is known and if the distance of the neutral point from the wire is mea­ sured, then the strength of the magnetic field set up by the current in the wire at the neutral point can be calculated. The horizontal component of the Earth’s magnetic field strength at the point in question is then known.