  This note considers two related topics regarding the {\it self-induced charge
current} in the experiments detecting photon rest mass: (i) the effective rest
mass, $(0.3\sim 1)\times 10^{-54}$ Kg, of photons arising from the ions in the
secondary cosmic ray is compared to Luo {\it et al.}'s new upper limit on
photon rest mass ($1.2\times 10^{-54}$ Kg); (ii) there are two main classes of
experiments to detect photon rest mass based respectively on the
Amp\`{e}re-Maxwell-Proca equation and the Lagrangian (or Hamiltonian) density
of electromagnetically interacting system.
