how does capacitance of finger/thumb work?

May 27, 2015 3:32 PM

I sliced my thumb in half last year. It's fine now, but touchscreens don't recognize touches using thumb. Why not?

A year ago, I had a kitchen knife accident and sliced most of the way through the pad of my thumb, right to the bone. The ER doc thanked me for making such a clean cut with such a sharp knife...it was no problem for her to stitch things together, and the wound healed up very nicely. The scar is barely visible and the only residual effect is a lack of feeling on the pad.

However, I have difficulty using the touchscreen of my iPhone with that thumb -- the screen does not properly register the touch of my finger. The touchscreen works just fine with any other finger and the other thumb. Why?

This does not impact my life in any way except as a trivial curiousity. I just like to understand everything to with science (whether biology, physics, electronics, whatever) from first principles. Appreciate if those with the relevant knowledge could shed some light on this.

Thanks.

posted by wutangclan to Science & Nature (2 answers total)

Best answer: Bummer. I have had a similar experience. Apparently the damaged area carries just enough less moisture to decrease conductivity and thus electrical capacitance. I recalled seeing a recent article by yet another person with this affliction, here. Perhaps a conductive cream (ha, zinc oxide?) might help. Homogeneous conductors have a skin effect which I expect would apply in a more complex way to literal skin. On the other hand, heh, there might be enough tissue damage below the surface to decrease conductivity. There exist capacitive gloves. More stylish, a stylus! DIY or buy. I have not yet tried those circumventions. The adequate expedient for me has been to use my pointer finger together with other fingers, or for some situations use my other hand.
posted by gregoreo at 4:40 PM on May 27, 2015

If it is a moisture thing -- try moistening the thumb a little bit and see if that helps.

Sometimes, when I have trouble with touchscreens, a little moisture does seem to help. I think my digits are relatively undamaged, but now that you mention this, I'm going to check for scars. I have cut my fingers in various incidents through the years, and I wonder if it has been a factor.
posted by amtho at 7:01 PM on May 27, 2015