:<math> I(l) = \sum_{n=1}^N a_n f_n(l) \mathbf{\hat{s}_n} </math>
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where l is the length coordinate along the wire with l=0 at its start point. <math>f_n(l)</math> is the scaled and translated version of the linear basis function <math>f(l)</math> shown in the previous figure. <math>\mathbf{\hat{s}_n}</math> is the unit vector along wire.