plasmonic ellipsoids [@BohrenHuffman]. By analogy with the electric response to electric driving quantified by $
\boldsymbol{\alpha}_{EE}$, the tensor $
\boldsymbol{\alpha}_{HH}$ quantifies the magnetic dipole induced by a driving magnetic field. Finally, the off-diagonal blocks represent magneto-electric coupling. The lower diagonal $ \boldsymbol{\alpha}_{HE}$ quantifies the magnetic dipole induced by an incident electric field, and $
\boldsymbol{\alpha}_{EH}$ quantifies the electric dipole induced by an incident magnetic field. Such magneto-electric coupling is well known to occur in the constitutive tensors of metamaterials [@Rockstuhl06]. Indeed, the first metamaterials consisted of split ring resonators, in which there is a magnetic response without any driving magnetic field in normal incidence experiments [@APLSoukoulis]. However, the relative strength of magneto-electric coupling in the polarizability, i.e., $
\boldsymbol{\alpha}_{EH}$, and $\boldsymbol{\alpha}_{HE}$ have not been experimentally quantified.

Electrodynamic Onsager relation
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There are several constraints on $\boldsymbol{\alpha}$. In addition to any symmetry of the scatterer itself that may impose zeros in the polarizability tensor, these constraints are due to reciprocity and to energy conservation. We start by examining the constraints imposed by reciprocity. It is well known from the field of bi-anisotropic materials [@Lindell_biisotropicbook] that reciprocity imposes so-called Onsager constraints on the most general constitutive tensors relating ($\bm{D},\bm{B}$) to ($\bm{E},\bm{H}$). Already 