One embodiment is directed to an electric plug having {HEAD ~ a central signal shaft} terminating in {TAIL ~ a tip}, wherein the tip has a substantially frustoconical nose, a convex annular shoulder centered at an axial distance of less than 0.120 inch from the front end of the tip, and a concave waist having a radius of curvature greater than about 0.75 inch, defining the detent adjacent to the bulbous portion 
One embodiment is directed to an electric plug having { HEAD ~ a central signal shaft } terminating in { TAIL ~ a tip } , wherein the tip has a substantially frustoconical nose , a convex annular shoulder centered at an axial distance of less than 0.120 inch from the front end of the tip , and a concave waist having a radius of curvature greater than about 0.75 inch , defining the detent adjacent to the bulbous portion
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