A novel experimental approach based on a toroid Cavendish balance is u
sed to evaluate the product of photon mass squared and the ambient cos
mic magnetic vector potential A. The method is based on the energy den
sity of the vector potential in the presence of photon mass, not on me
asurement of the magnetic field. The experiment discloses A mu(gamma)(
2) < 2 X 10(-9) T m/m(2), with mu(gamma)(-1) as the characteristic len
gth associated with photon mass. Consequently, if the ambient magnetic
vector potential is A approximate to 10(12) T m due to cluster level
fields, mu(gamma)(-1) > 1 x 10(10) m. If we conservatively use galacti
c fields prior to a reversal, then mu(gamma)(-1) > 1 x 10(9) m, a figu
re still superior to that derived from the Jovian magnetic field.