Experimental Limits on the Photon Mass and Cosmic Magnetic Vector Potential

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 Aμ_γ²<2×10^-90ex0exTm/m², with μ_γ^-1 as the characteristic length associated with photon mass. Consequently, if the ambient magnetic vector potential is A≈10¹²0ex0exTm due to cluster level fields, μ_γ^-1>2×10¹⁰0ex0exm. If we conservatively use galactic fields prior to a reversal, then μ_γ^-1>1×10⁹0ex0exm, a figure still superior to that derived from the Jovian magnetic field.