The focussing properties of crossed electric and magnetic fields for electrons have been investigated for the case of circular orbits, and it has been found that this combination of fields provides extremely sharp focussing, which is in very good agreement with the theoretical predictions. Measurements of the electric field intensity have revealed the presence of polarization layers which form on the plates of the electric field. These layers reduce the effective potential which is used to deflect the electron beam. The absolute magnitude of these layers has been measured for gold and bronze, and it has been found to depend upon; (a) material of the plates, (b) gas pressure, and (c) electron intensity. These layers are found to be constant under a fixed set of conditions and to vary in a reversible manner as conditions are altered. In addition, permanent insulating layers may be formed if electrons bombard a metal surface. Errors in $\frac{e}{m}$ determinations may be traced to these layers and tests are developed for their elimination.

• Received 8 September 1933

DOI:https://doi.org/10.1103/PhysRev.44.1006