By a study of the deflection of a beam of CsI molecules in a homogeneous electric field an upper limit is established for the charge on a CsI molecule. This upper limit is 1.7×${10}^{-22\mathrm{}}$ esu or 4×${10}^{-13\mathrm{}}$ of the electronic charge, and implies that the magnitude of the electron-proton charge difference is less than 3×${10}^{-15\mathrm{}}$ of the electronic charge or that the charge on a neutron is less than 2×${10}^{-15\mathrm{}}$ of the electronic charge.

• Received 22 August 1956

DOI:https://doi.org/10.1103/PhysRev.105.170