In passing through a mall, hospital, or doctors office nowadays you may often see, mounted on a wall or on a desk, a solitary monitor lacking a mouse or keyboard. These devices are operated through the screens, which can sense exactly where they've been touched and react accordingly. These devices are called 'Touch-sensitive Monitors', and come in three variants.

• Resistive
• Capacitive
• Surface Acoustic Wave

Resistive Monitors - an Economist's Friend

The resistive monitor is the most common variety, and nearly all touch-sensitive monitors in public places use this type. Resistive monitors are composed of a glass plate or panel, covered in a conductive and a resistive metallic layer. The two metallic layers are held apart by spacers, with a scratch-resistant layer covering the whole of it. During operation, an electric current runs through the metallic layers. When someone touches the screen, the two layers come into contact. The computer notes the change in the electric field and calculates the position of the touch. Once this position is known, the program that interprets these signals tells the computer what they mean. This is the system commonly used in palm pilots and other PDA systems, as the signal is sent from any form of contact, such as a finger or 'stylus'. Unfortunately, the screen can be damaged with sharp objects, but this is often made up for by its low production and sale cost.

The Capacitive - Clearer than Resistive

The next variety of touch sensitive monitor is the capacitive. In the capacitive system, an electrical charge is stored on the glass screen of the monitor. When you touch the glass of the screen, charge from the screen is transferred to the person touching the screen, and the charge of the screen decreases. The computer senses this and calculates, using the relative differences in charge in each corner to work out where the touch occurred, and sends that information back to the screen software. A benefit to using this form of touch-screen technology is that, unlike the resistive idea, which transmits only 75 percent of the light it should, capacitive touch-screens provide up to ninety percent, giving a much clearer picture.

Acoustic Wave: The Best of The Best

The last system is the surface acoustic wave system. It consists of the same monitor, and two transducers, one receiving and one sending. These are placed along the x and y axes of the monitor's glass plate. Also on the glass are reflectors, devices which reflect electric signals sent from one transducer to the other. The system allows for instantaneous reactions, as the receiving transducer is able to tell immediately if a wave has been disturbed. This system also has one hundred percent light and clarity, and is good for displaying detailed graphics. This is the best system, but is by far the most expensive.