How vandals destroy a touch-screen.

[benburch]

Almost all modern touch-screens are of two types, capacitive glass and resistive glass/plastic sandwiches.

In the capacitive glass touch-screens, the capacitive layer is a film laid over the glass of a CRT or LCD panel, and which is glued directly to that panel to form a hard surface. It is divided into a series of rectangular regions that constrain the placement of on-screen buttons. In addition, capacitive touch-screens cannot be used with a gloved hand, and will also not work in very dry, cold weather where there is little sweat on the fingertip to provide a capacitive coupling to the active layer.

In the resistive touch-screen, the touch-screen mechanism is a glass plate and plastic film layer that us usually just laid over the display screen. The plastic layer is above the glass plate. and requires only pressure at a particular point to complete the circuit which is read out as pair of voltage drops that give an X-Y coordinate. Both the glass layer and the plastic layer are coated with a vapor-deposited layer of Indium-tin-oxide (ITO). This type is much more flexible in its ability to put an active touch region anywhere on the screen and is what most designers prefer. It has its problems though, as it is used, the ITO can get scrubbed off the film layer by people using too much stylus pressure, and also if you exert pressure on two parts of the screen, for example if you lay the side of your hand on the screen while using the stylus, the readout will be moved to a different position than you intended. Also, this sort of touch-screen needs to be calibrated before use, and if the calibration is incorrect will sometimes move touches into places you never intended, and could even cause you to poke one button and have the result be activation of a totally different button on the screen. If you knew what you were doing, you could mis-calibrate such a machine to systematically read out in the wrong spots.

In either sort, the film layer is the achilles heel, and vandals often destroy such machines by scoring the very edge of the screen right at or under the bezel with a sharp object such as a ring or key, destroying the conductivity of the film layer and rendering the machine useless.

Were this done to voting machines by an organized vandalism campaign, the results would be terrible; If the vandals were early voters, nobody could vote on that machine for the remainder of that election day, Were this to be done in districts that favor a particular party, then it is likely that party would lose the election.

So, next primary election, those of you in Democratic districts ought to watch carefully for this sort of organized vandalism.

You've been warned!