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We study the Sun's magnetic field with an instrument called a vector magnetograph.  To understand what a vector magnetograph measures, we must first understand what a vector is.  A vector is simply a quantity which has magnitude and direction.  For instance, someone who says she has traveled at 55 mph (the magnitude) toward the northeast (the direction) has expressed a vector quantity.  

So,  to have all the information about a magnetic field, we need to know how big it is (its magnitude) and in what direction it's pointing.  The figure below shows iron filings distributed about a dipole (two poles) magnet.  The iron filings trace out the field of the magnet and demonstrate how magnetic field has a direction. 

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