Archimedean spiral

Three 360° turnings of one arm of an Archimedean spiral

The Archimedean spiral (also known as the arithmetic spiral) is a spiral named after the 3rd century BC Greek mathematician Archimedes. It is the locus of points corresponding to the locations over time of a point moving away from a fixed point with a constant speed along a line which rotates with constant angular velocity. Equivalently, in polar coordinates (r, θ) it can be described by the equation

with real numbers a and b. Changing the parameter a will turn the spiral, while b controls the distance between successive turnings.

Archimedes described such a spiral in his book On Spirals.

Characteristics

This Archimedean spiral is distinguished from the logarithmic spiral by the fact that successive turnings of the spiral have a constant separation distance (equal to 2πb if θ is measured in radians), while in a logarithmic spiral these distances form a geometric progression.

Note that the Archimedean spiral has two arms, one for θ > 0 and one for θ < 0. The two arms are smoothly connected at the origin. Only one arm is shown on the accompanying graph. Taking the mirror image of this arm across the y-axis will yield the other arm.

One method of squaring the circle, by relaxing the strict limitations on the use of straightedge and compass in ancient Greek geometric proofs, makes use of an Archimedean spiral.

Sometimes the term Archimedean spiral is used for the more general group of spirals

The normal Archimedean spiral occurs when x = 1. Other spirals falling into this group include the hyperbolic spiral, Fermat's spiral, and the lituus. Virtually all static spirals appearing in nature are logarithmic spirals, not Archimedean ones. Many dynamic spirals (such as the Parker spiral of the solar wind, or the pattern made by a Catherine's wheel) are Archimedean.

Applications

Mechanism of a scroll pump

The Archimedean spiral has a plethora of real-world applications. Scroll compressors, made from two interleaved Archimedean spirals of the same size, are used for compressing liquids and gases.^[1] The coils of watch balance springs and the grooves of very early gramophone records form Archimedean spirals, making the grooves evenly spaced and maximizing the amount of music that could be fit onto the record (although this was later changed to allow better sound quality).^[2] Asking for a patient to draw an Archimedean spiral is a way of quantifying human tremor; this information helps in diagnosing neurological diseases. Archimedean spirals are also used in DLP projection systems to minimize the "Rainbow Effect", making it look as if multiple colors are displayed at the same time, when in reality red, green, and blue are being cycled extremely quickly.^[3] Also, Archimedean spirals are used in food microbiology to quantify bacterial concentration through a spiral platter.^[4]

See also

• Clackson scroll formula
• Hyperbolic spiral
• Fermat's spiral
• Logarithmic spiral
• Triple spiral symbol

References

1. ^ Sakata, Hirotsugu and Masayuki Okuda. "Fluid compressing device having coaxial spiral members". http://www.freepatentsonline.com/5603614.html. Retrieved 2006-11-25. 
2. ^ Penndorf, Ron. "Early Development of the LP". http://ronpenndorf.com/journalofrecordedmusic5.html. Retrieved 2005-11-25. . See the passage on Variable Groove.
3. ^ Wilson, Tracy V.. "Adding Color and the Reliability of DLP". http://electronics.howstuffworks.com/dlp1.htm. Retrieved 2005-11-25. 
4. ^ J. E. Gilchrist, J. E. Campbell, C. B. Donnelly, J. T. Peeler, and J. M. Delaney. "Spiral Plate Method for Bacterial Determination". http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=4632851. 

External links

• Weisstein, Eric W., "Archimedes' Spiral" from MathWorld.
• archimedean spiral at PlanetMath.
• Page with Java application to interactively explore the Archimedean spiral and its related curves
• Online exploration using JSXGraph (JavaScript)