Fermat's Spiral

Polar equation: r^2 = a^2theta

Click below to see one of the Associated curves.

Definitions of the Associated curves

Evolute       Involute 1       Involute 2

 Inverse curve wrt origin      Inverse wrt another circle

Pedal curve wrt origin      Pedal wrt another point

Negative pedal curve wrt origin      Negative pedal wrt another point

   Caustic wrt horizontal rays      Caustic curve wrt another point

This spiral was discussed by Fermat in 1636.

For any given positive value of theta there are two corresponding values of r, one being the negative of the other. The resulting spiral will therefore be symmetrical about the line y = -x as can be seen from the curve displayed above.

The inverse of Fermat's Spiral, when the pole is taken as the centre of inversion, is the spiral r^2 = a^2/theta.

For technical reasons with the plotting routines, when evolutes, involutes, inverses and pedals are drawn only one of the two branches of the spiral are drawn.


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JOC/EFR February 1996