Projected capacitive technologies detect touch by measuring the capacitance at each addressable electrode. When a finger or a conductive stylus approaches an electrode, it disturbs the electromagnetic field and alters the capacitance. This change in capacitance can be measured by the electronics and then converted into X,Y locations that the system can use to detect touch.
3M Project Capacitive Technology (3M PCT) is based on mutual capacitance to create multi-touch interactive. Mutual capacitance is the intentional or unintentional capacitance between two "charge holding objects” (see illustration). Projected capacitance touchscreens intentionally create mutual capacitance between elements of columns and rows in the vicinity where each intersect the other. This allows the system electronics to measure each node (intersection) individually to detect multiple touches on the screen during one screen scan.
When a finger touches near an intersection, some of the mutual capacitance between the row and column is coupled to the finger which reduce the capacitance at the intersection as measured by the system electronics. This reduced capacitance crosses the "touch threshold" set by the electronics indicating a touch has occurred.