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In mathematics, a spiral is a curve which emanates from a central point, getting progressively farther away as it revolves around the point. == Spirals and helices == Two major definitions of "spiral" in a respected American dictionary are:〔"(Spiral )", ''The American Heritage Dictionary of the English Language'', Houghton Mifflin Company, Fourth Edition, 2009.〕 : a. A curve on a plane that winds around a fixed center point at a continuously increasing or decreasing distance from the point. b. A three-dimensional curve that turns around an axis at a constant or continuously varying distance while moving parallel to the axis; a helix. Definition a describes a planar curve, that extends in both of the perpendicular directions within its plane; the groove on one side of a record closely approximates a plane spiral (and it is by the finite width and depth of the groove, but ''not'' by the wider spacing between than within tracks, that it falls short of being a perfect example); note that successive loops ''differ'' in diameter. In another example, the "center lines" of the arms of a spiral galaxy trace logarithmic spirals. Definition b includes two kinds of 3-dimensional relatives of spirals: * A conical or volute spring (including the spring used to hold and make contact with the negative terminals of AA or AAA batteries in remote controls), and the vortex that is created when water is draining in a sink is often described as a spiral, or as a conic helix. * Quite explicitly, b also includes a cylindrical coil spring and a strand of DNA, both of which are quite helical, so that "helix" is a more ''useful'' description than "spiral" for each of them; in general, "spiral" is seldom applied if successive "loops" of a curve have the same diameter.〔 In the side picture, the black curve at the bottom is an Archimedean spiral, while the green curve is a helix. The curve shown in red is a conic helix. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「spiral」の詳細全文を読む スポンサード リンク
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