Precision methods used in constructing electric wave filters for carrier systems

Electric wave filters are used extensively in carrier telephone and telegraph systems. In order that such systems may be operated efficiently and economically, the requirements placed on the filters they employ are occasionally so severe in nature that new methods in design or construction must be developed to make the commercial production of the filters possible. The band filters for the Type “C” Carrier Telephone System are cases in point. This paper sets forth the requirements which were met in the design of the filters for this system, and describes a new manufacturing adjustment made necessary by these requirements. This feature consists essentially of an inductance continuously variable over a small range above and below its nominal value; the adjustment is not used to set the coil inductance at its specified value, but to locate correctly the series or parallel resonance of the mesh of which the coil is a part. The bridge and associated apparatus developed to facilitate this adjustment are also described.