Cómo funcionan las pantallas táctiles

Incluida la pantalla multipunto del iPhone

Howstuffworks "How the iPhone Works
"

Most of the time, these systems are good at detecting the location
of exactly one touch. If you try to touch the screen in several places
at once, the results can be erratic. Some screens simply disregard all
touches after the first one. Others can detect simultaneous touches,
but their software can't calculate the location of each one accurately.
There are several reasons for this, including:

• Many systems detect changes along an axis or in a specific direction instead of at each point on the screen.
• Some screens rely on system-wide averages to determine touch locations.
• Some
  systems take measurements by first establishing a baseline. When you
  touch the screen, you create a new baseline. Adding another touch
  causes the system to take a measurement using the wrong baseline as a
  starting point.

  Multi-touch Systems

  
  

  To allow
  people to use touch commands that require multiple fingers, the iPhone
  uses a new arrangement of existing technology. Its touch-sensitive
  screen includes a layer of capacitive
  material, just like many other touch-screens. However, the iPhone's
  capacitors are arranged according to a coordinate system. Its circuitry
  can sense changes at each point along the grid. In other words, every
  point on the grid generates its own signal when touched and relays that
  signal to the iPhone's processor. This allows the phone to determine
  the location and movement of simultaneous touches in multiple
  locations. Because of its reliance on this capacitive material, the
  iPhone works only if you touch it with your fingertip -- it won't work
  if you use a stylus or wear non-conductive gloves.