A NEW LIMIT ON PHOTON MASS has been established using a tabletop apparatus. The mass of photons, the corpuscular manifestation of light, is usually assumed to be zero. A non- zero photon mass would have several implications, such as a frequency-dependent speed for light and the existence of longitudinal electromagnetic waves (in the conventional waves we know, the electric and magnetic fields oscillate perpendicularly to the line of travel). The best previous limit on photon mass (using a reasonably connected line of inference) comes from observations of Jupiter's magnetic field. The value, 6 x 10^-16 eV, was determined in 1975. Now, Roderic Lakes of the University of Wisconsin (608-265-8697) has tightened the limit further---to below 7 x 10^-17 eV---by carefully watching for anomalous torques in the motion of a Cavendish balance, basically a steel toroid wrapped in current-carrying coils. Essentially, Lakes' novel approach is to seek a cosmic vector potential. In electromagnetism, the change in the vector potential (denoted by the letter A) is related to the strength of the magnetic field in that part of space, just as an electric field can be calculated from a changing electric potential (voltage). Normally, A cannot be measured point-for-point in space, but it could be if the photon's mass were nonzero. Lakes did not find a nonzero A, but his method might be useful (beyond setting a new limit on photon mass) for searching out new features of short-range forces, such as the nuclear strong force. (Physical Review Letters., 2 March.) Physical Review Letters, 2 March 1998.)