Testing the neutrality of matter by acoustic means in a spherical resonator

Measurements to test the neutrality of matter by acoustic means are reported. The apparatus is based on a spherical capacitor filled with gaseous SF_6 excited by an oscillating electric field. The apparatus has been calibrated measuring the electric polarizability. Assuming charge conservation in the β decay of the neutron, the experiment gives a limit of epsilon_{p−e} <= 1 × 10−21 for the electron-proton charge difference, the same limit holding for the charge of the neutron. Previous measurements are critically reviewed and found to be inaccurate.