An Archimedean Spiral is also known as an arithmetic spiral
since there is a constant separation distance on successive turnings of the spiral.
It is the trace of a point moving away from a starting point with a constant speed
along a line which rotates with constant angular velocity.
In the formulas used here, b t cos(t + a), b t sin(t + a),
b controls the “tightness” of the spiral,
a changes its orientation, while t is the parameter.
The slider N controls how many turnings there are.
