this article, we will make use of an A’Campo formula in the more general context of a Cartier divisor on a normal surface. In fact, the notion of monodromy eigenvalues can be generalized in a straightforward way to any ideal sheaf ${\mathcal I}$ on a general variety $X$. Therefore, we state the formula in the following general context. Let ${\mathcal I}$ be a sheaf of ideals on a variety $X$, let $Y := V({\mathcal I})$ be the associated subscheme in $X$ and suppose that $\text{Sing}(X) \subset Y$. Consider the blow-up $\sigma: X' \rightarrow X$ of $X$ with center $Y$ and let $E'$ be its exceptional divisor, that is, the inverse image $\sigma^{-1}(Y)$ (with its non-reduced scheme structure). One can show that $E'$ is the projectivization $P(C_YX)$ of the normal cone $C_YX$ of $Y$ in $X$. Denote the corresponding projectivization map by $p:C_YX \rightarrow E' = P(C_YX)$. For a point $e\in E'$, we define the monodromy eigenvalues of ${\mathcal I}$ at $e$ as the eigenvalues of the monodromy transformations $\tilde{M}^k_y$ for some $y \in C_Y{\mathbb C}^{n+1}\setminus Y$ mapped to $e$ under $p$; this is independent of the choice of $y$. Hence, we can define the *zeta function of monodromy* of ${\mathcal 