Simple Way to Measure Magnetic Fields The induced emf in the loop is measured to be V. What is the magnitude B of the magnetic field that the loop was in? Express your answer in terms of some or all of the variables A, C, and V. A loop of wire is at the edge of a region of space containing a uniform magnetic field B. The plane of the loop is perpendicular to the magnetic field. Now the loop is pulled out of this region in such a way that the area A of the coil inside the magnetic field region is dA decreasing at the constant rate c. That is, -C, with c > 0.  For the case of a square loop of side length L being pulled out of the magnetic field with constant speed v (see the figure), what is the rate of change of area dA с