The invention relates to a coil apparatus of an oscillation sensor or exciter of a measuring transducer or a measuring device for measuring a density or mass flow of a medium flowing through a measuring tube of the measuring transducer or measuring device, comprising: a circuit board having a circuit board layer, at least one coil registering or producing a time varying magnetic field, wherein the coil has a winding region and a central region lacking turns of a winding, wherein the central region of a coil has a rectangular shape with oppositely lying, first sides and with oppositely lying, second sides, wherein the first sides have a first side length, and wherein the second sides have a second side length, wherein the electrically conductive trace has a trace breadth of at least 30 micrometer, wherein a ratio of first side length to second side length is greater than 3.25.