Hyperbolic spiral

It can be described in polar coordinates [math](r, \theta)[/math] by the equation

[math]r = \frac{a}{\theta}\,[/math]

with a real number [math]a[/math].

The JavaScript code to produce this picture

 var board = JXG.JSXGraph.initBoard('jxgbox', {boundingbox: [-10, 10, 10, -10]});
 var a = board.create('slider', [[0,5],[8,5],[0,4,30]],{name:'a'});
 var c = board.create('curve', [function(phi){return a.Value()/phi; }, [0, 0],0, 8*Math.PI],
             {curveType:'polar', strokewidth:4});