We determine the general structure of the partition function of the $q$-state Potts model in an external magnetic field, $Z(G,q,v,w)$ for arbitrary $q$, temperature variable $v$, and magnetic field variable $w$, on cyclic, Mobius, and free strip graphs $G$ of the square (sq), triangular (tri), and honeycomb (hc) lattices with width $L_y$ and arbitrarily great length $L_x$. For the cyclic case we prove that the partition function has the form $Z(Lambda,L_y times L_x,q,v,w)=sum_{d=0}^{L_y} tilde c^{(d)} Tr[(T_{Z,Lambda,L_y,d})^m]$, where $Lambda$ denotes the lattice type, $tilde c^{(d)}$ are specified polynomials of degree $d$ in $q$, $T_{Z,Lambda,L_y,d}$ is the corresponding transfer matrix, and $m=L_x$ ($L_x/2$) for $Lambda=sq, tri (hc)$, respectively. An analogous formula is given for Mobius strips, while only $T_{Z,Lambda,L_y,d=0}$ appears for free strips. We exhibit a method for calculating $T_{Z,Lambda,L_y,d}$ for arbitrary $L_y$ and give illustrative examples. Explicit results for arbitrary $L_y$ are presented for $T_{Z,Lambda,L_y,d}$ with $d=L_y$ and $d=L_y-1$. We find very simple formulas for the determinant $det(T_{Z,Lambda,L_y,d})$. We also give results for self-dual cyclic strips of the square lattice.
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