A novel experimental approach based on a toroid Cavendish balance is used to evaluate the product of photon mass squared and the ambient cosmic magnetic vector potential $A$. The method is based on the energy density of the vector potential in the presence of photon mass, not on measurement of the magnetic field. The experiment discloses $A{\mu }_{\gamma }^2<2\mathrm{}\times {10}^{-9\mathrm{}}\mathrm{T}\mathrm{m}/{\mathrm{m}}^2$, with ${\mu }_{\gamma }^{-1\mathrm{}}$ as the characteristic length associated with photon mass. Consequently, if the ambient magnetic vector potential is $A\approx {10}^{12}\mathrm{T}\mathrm{m}$ due to cluster level fields, ${\mu }_{\gamma }^{-1\mathrm{}}>2\mathrm{}\times {10}^{10}\mathrm{m}$. If we conservatively use galactic fields prior to a reversal, then ${\mu }_{\gamma }^{-1\mathrm{}}>1\mathrm{}\times {10}^9\mathrm{m}$, a figure still superior to that derived from the Jovian magnetic field.

• Received 2 June 1997

DOI:https://doi.org/10.1103/PhysRevLett.80.1826