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In projected capacitative touch screens, sensors measure change in electric field coupling from the drive electrode to sense/receive electrode. enter image description here

But don't field lines only terminate on conductive surfaces? This article mentions that the change in capacitance read-out when a finger is brought close to the sensor is of the order of a pico-farad. Considering how nonconductive the epidermis is (~100KOhms, worst case), this change is quite large. Wouldn't it be orders of magnitude lesser if epidermis was really acting as a sink? Could it be possible that the field lines actually terminate on conductive layers underneath the epidermis?

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    \$\begingroup\$ Why you get electrocuted if the skin is very non-conductive? \$\endgroup\$
    – Marko Buršič
    Nov 22, 2015 at 19:37
  • \$\begingroup\$ There are always charges on the human body mainly due to friction (ever notice the crackling sound sometimes when you remove a sweater) .They may even be of the order of kV . You may fry a CMOS IC by simply touching it due to ESD(Electrostatic Discharge) . \$\endgroup\$
    – user50456
    Nov 22, 2015 at 19:59
  • \$\begingroup\$ The electric field lines are bent by the charges on the human body which can go nowhere since, as you mentioned ,it has a resistance of >100k . \$\endgroup\$
    – user50456
    Nov 22, 2015 at 20:02
  • \$\begingroup\$ Say someone with quite nonconductive dry skin, then the dry skin would be just another thin layer of nonconductive dielectric on top of the nonconductive dielectric (glass?) panel. Then beyond this very thin layer of dry skin is probably relatively conductive. \$\endgroup\$
    – rioraxe
    Nov 23, 2015 at 8:03
  • \$\begingroup\$ @MarkoBuršič Electrocution occurs at high voltages, if touch-screens used such high AC drive signals, we'd all probably be dead. \$\endgroup\$
    – user1155386
    Nov 23, 2015 at 8:34

3 Answers 3

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It's a capacitive touchscreen (not resistive) and it uses AC pulses to detect the capacitance of the human body. The thin layer of outer skin on the body may indeed be quite non-conductive but the glass which the field projects thru is hundreds (or thousands) of times more non-conductive so clearly the skin's low conductivity isn't of vital importantance in this scenario.

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I'm not sure where did you get the 100kOhm epidermis resistance, but I guess this number is valid only for low-voltage DC. Touchscreens work with AC as the article you cited confirms. AC is carried though capacitors, so it largely ignores the epidermis when it travels through the body (think of epidermis as of yet another capacitor shunted by a 100kOhm resistor). The internal body resistance is considered to be much smaller, around 1.5kOhm or so.

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Could it be possible that the field lines actually terminate on conductive layers underneath the epidermis?

In other words you're asking if the rest of finger (beyond the epidermis), and more, the rest of the body has an impact on the capacitance change of the touch screen, right? If so, the answer is yes. You could experiment it your self, although it may hurts a bit. Cut a very thin slice of your finger extremity and put it on a touch screen. You would see, it won't work ;-)

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