If the solenoid has length \(\ell\), the quantity \(n\) in Equation 19-17 equals the total number of windings \(N\) divided by \(\ell\). We can then write the magnetic flux through the crosssectional area \(A\) of each winding of the solenoid as
\(\Phi_B = BA = (\mu_0 ni)A = \left(\frac{\mu_0Ni}{\ell}\right)A\)
.