  We investigate in a quantitative way the plasmon resonance at eigenvalues and
the essential spectrum (the accumulation point of eigenvalues) of the
Neumann-Poincar\'e operator on smooth domains. We first extend the
symmetrization principle so that the single layer potential becomes a unitary
operator from $H^{-1/2}$ onto $H^{1/2}$. We then show that the resonance at the
essential spectrum is weaker than that at eigenvalues. It is shown that
anomalous localized resonance occurs at the essential spectrum on ellipses, but
cloaking does not occur on ellipses unlike the core-shell structure considered
in [20]. It is shown that resonance does not occur at the essential spectrum on
three dimensional balls.
