{HEAD ~ One embodiment} is directed to {TAIL ~ an electric plug} having a central signal shaft terminating in 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 
is directed to
{ HEAD ~ One embodiment } is directed to { TAIL ~ an electric plug } having a central signal shaft terminating in 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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