Science Questions

How do mouse pads work?

Sun, 23rd Sep 2007

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Question

Brian, Chicago asked:

This summer I've taken up playing guitar. Unfortunately, all I have is a steel-stringed guitar and it is forming some pretty wicked calluses on my fingers. I've noticed that the little mouse pad on my laptop no longer responds to the tips of my callused fingers. It responds perfectly with the fingerprints of all fingers and with the tips of my right hand. The click-wheel of my ipod responds to my callused fingers. What gives? What is it about the mouse pad on my computer that leads it to no longer acknowledges the existence of my left finger tips?

Answer

I think that trackpads on laptops work by having two sets of wires, one running horizontally and one vertically, and they they look at the capacitance between the two of them - so if you make a voltage with one of them, how much is that voltage picked up by the other one.  If you put your finger near it, this changes quite considerably, in fact if you put anything conductive near it it will change.  The problem is that if you've got thick, dry, calloused skin on the tip of your finger, this acts as an insulator and stops this effect from happening.  The ipod click wheel is either more sensitive, or possibly pressure sensitive.

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