permittivity of the core, find H12, H23, H34, and H41(H12 denotes the magnetic field intensity in the branchPP2 and others are similarly defined). \text { Verify that } \Im=H_{12} l_{2}+H_{23} l_{1}+H_{34} l_{2}+H_{41} l_{1} \text {. } \text { A more general equation is } \Im=\sum H_{m} l_{m} \text { and it is appli- } cable to any closed path.
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