vary the frequency of the wave.
They have a relatively low resistance, being made partially, as they are, of steel. In the range of 2-7Kohms, which is a great deal less than the POT's I was originally using. This means that I am somewhat stuck with the squeaky/screechy end of the waveform and can't get any of those deep rumbles or farting sounds I love so much.
It's fun anyway. Like I say, I'm not sure for what, but it gives me a much more tangible way of measuring the variable resistance of various fabrics and so on