| Summary: | We formulate a linear response theory of the chiral magnetic effect in a finite Weyl semimetal, expressing the electrical current density j induced by a slowly oscillating magnetic field B or chiral chemical potential μ in terms of the scattering matrix of Weyl fermions at the Fermi level. Surface conduction can be neglected in the infinite-system limit for $\delta j/\delta \mu $ , but not for $\delta j/\delta B$ : the chirally circulating surface Fermi arcs give a comparable contribution to the bulk Weyl cones no matter how large the system is, because their smaller number is compensated by an increased flux sensitivity. The Fermi arc contribution to ${\mu }^{-1}\delta j/\delta B$ has the universal value ${(e/h)}^{2}$ , protected by chirality against impurity scattering—unlike the bulk contribution of opposite sign.
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