Overvoltage protector for telephone lines
 An overvoltage or surge protector, particularly for telephone lines, has two electrodes in opposition in a housing. The gap between the inner ends of the electrodes is set by a layer of dielectric material on the center portion of one or both end surfaces of the electrodes. Breakdown occurs with assistance from dielectric stimulation of the arc, resulting from an electrical field across the dielectric. The center portions are surrounded by annular grooves, which are also covered with a dielectric layer. The end surfaces external to the center portions can be convex, and also roughened. 
Northern Telecom Limited
H H H
H01T H01T H01T
inventional inventional inventional
Frederick C. James E. John D.
Livermore Anderson Lee