[4] Williams E R, Faller J E, Hill H A. New experimental test of Coulomb's law: a laboratory upper limit on the photon rest mass [J]. Physics Review Letters, 1971, 26(12): 721-724.
A high-frequency test of Coulomb's law is described. The sensitivity of the experiment is given in terms of a finite photon rest mass using the Proca equations. The null result of our measurement expressed in the form of the photon rest mass squared is μ2 = (1.04 ± 1.2) × 10-19 cm-2. Expressed as a deviation from Coulomb's law of the form 1/(r 2+q), our experiment gives q = (2.7 ± 3.1) × 10-16. This result extends the validity of Coulomb's law by two orders of magnitude.