In the extreme case that b = 0 ( É = Ï \/ 2 ) the spiral becomes a circle of radius a. Conversely , in the limit that b approaches infinity ( É â ' 0 ) the spiral tends toward a straight line . In the extreme case that the spiral becomes a circle of radius . Conversely , in the limit that approaches infinity ( Ï â ' 0 ) the spiral tends toward a straight half-line .