Touch screens are
everywhere: smartphones, laptop screens, supermarket checkouts, restaurant tills, ATMs, airport check-in kiosks, museum information booths and GPS devices, to name
a few.
Hand-held devices and computer screens may be the latest to use the technology, but the basics have been around for a long time. Touch screens were invented in the 1970s but were too expensive to produce commercially until early this century when Bell Laboratories cooked up the right materials.
The concept is even older.
In the 1950s Otis Elevator Co. purportedly invented and implemented the
nonmoving sensor — nonmoving as opposed to mechanical buttons like keyboard keys or the push buttons still in use on some elevators.
In my long-lost youth those elevator call “buttons” with the eerie orange rings that illuminate when your finger touches their center were mysterious if not magical.
After years of studying physics, it is still quite mysterious to me, although somewhat less magical.
Like today’s touch screens, those buttons were capacitive sensors, but the technology today is vastly more complex. Capacitance is the ability to store electric charge on a conductive surface of a certain area. It is based on a fundamental physical law that is the basis for everything we understand about electricity: Electric charges repel.
Electrons, which carry a single negative charge, will arrange themselves as far apart as possible on the surface of a conductor due to their repulsive forces on
one another. To push more electrons onto the conductor, work must be done against their mutually repulsive forces. Mechanically, this is analogous to work that must be done against the elastic force of a balloon as it inflates.
In both cases the work is stored as potential energy. In the balloon there is pressure. On the capacitor there is voltage, which is the average electrical potential per charge.
The typical capacitive touch screen has a coating of indium tin oxide on one side of a piece of transparent plastic that is sandwiched between the display and the surface of the flat-screen monitor. Indium tin oxide is used because it is relatively inexpensive, electrically conductive and transparent to visible light.
Touching the screen disturbs the electric field in the membrane because human skin is a conductor that stores varying amounts of electrical charge depending on the surroundings, and so has a natural capacitance. That changes the capacitance where the finger touches.
A common technology known as projective capacitance touch uses an X-Y grid etched on the indium tin oxide layer. In these systems the finger distorts the capacitance field of the array like lightly poking one’s skin. The processor chip scans the grid to locate the position on the screen.
The grid has high resolution and does not require direct contact with the coating, so the conducting layers can be covered with protective insulating layers and operate even under screen protectors. It also enables the use of more than one finger to “pinch” an image to shrink it, or to swipe the screen and watch with wonder as the image continues to move on the screen with its own capacitive inertia.
Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.
