Probe Test

What follows are the results of unbelievably rigorous iPod touch tests that were designed to identify a viable non-finger touchscreen probe. There was a time when these devices (and their larger successors) were not switch accessible through the 30-pin port, so I was looking at the possibility of triggering the touchscreen with a tap from a solenoid tipped with some type of effective material. (Target area selection would have involved a subsequent project.) Improved accessibility methods over the last several years (finally appearing with iOS 7) has made this information moot, but that lack of immediate relevance is no different than in any treatise that somberly documents the History of Science.

To begin with, the probe surface contact area had to be at least 4.5 mm in diameter; that is to say, 4.5 mm (or more) along only one axis wouldn’t work.

The tip of an alkaline battery worked (when held flat), live or dead, AAA through D, except for the AAA positive tip (which was too small). The cylindrical can is drawn steel.

A (live) LR44 battery, however, did not work. I don’t know why. Neither did a (live) SR44 battery. Neither did a (live) CR2016 lithium battery. I don’t know what their case construction is. They work if you’re touching them, so it’s not a matter of too much surface area.

In fact, disks cannibalized from hard drives (some quite old) worked when held flat, so the probe’s maximum surface area is only constrained by pragmatics.

The grip (not the tip) of stainless steel lab tweezers worked (both 10 mm and 7.5 mm).

The head of a 6 mm nail worked, but it had to be carefully placed to make flat contact.

Drill bit bases (probably low-carbon steel) did not work. (But they were great for measuring the probe minimum surface area when they had skin contact.)

Fifteen sample metal fabrics from LessEMF worked. (The only two that did not work, Ex-Static and NatureShield, look more like a loose metal grid woven into the fabric.)

Thinly sliced deli ham worked. (“Why test ham?”, you might ask. “Well,” I might answer, “it’s perfectly logical: I didn’t have a hot dog handy.”)

Someone said a damp sponge would work, and while it did not work when I tested it, it did clean the screen nicely. I didn’t have natural sponge handy to test, so maybe that’s the difference.

A shiny brass-lookin’ business card case worked.

Coins of many nations, even when smoothly worn into obscurity, did not work.

I did not have a piece of conductive silicone pad big enough to test, but that might be nicer to the glass.

A very powerful rare earth magnet… was not tested on the device because I have these rare flashes of common sense (that I occasionally heed). Part of my caution in this case was that I did not own the device that I used for the test, so someone else would have to muck about with other magnets (which are reported to be good candidates).

Speaking of which, in case anyone wants to try to reproduce these results, or test other types of materials (you know who you are), I decoupled these test items from skin capacitance by wrapping my fingertips in adhesive bandage strips, then gripped the items with additional cloth padding. I had tried tape in double layers (first masking, then electrical), then I tried a pair of vinyl gloves, but when none of that worked the bandages were simply easier (i.e., a readymade tape+cloth unit).

Just for comparison I tried this stuff out on a couple of capacitive touchpads as well, but they were much less willing to accept finger substitutes. Maybe they use surface rather than projected capacitance.

I simply don’t know.

Further research is suggested.

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