How Do Capacitive Touch Screens Work?

Recently a forum post surfaced where someone reported using different objects to control their cellphone. I have no idea why but they decided to see if a hotdog could substitute in for fingers. Perplexed that it worked, they then tried various other objects around their kitchen. Some worked, some didn’t.

Let’s look at the physics behind why you can use a hotdog to control your cellphone (while ignoring the question of why you’d want to).

Touch screens function based on a electrical principle called “capacitive touch.” Basically, holding your finger on or very close to the screen results in the screen reporting a touch or swipe action. In Android OS, this goes to a Java event listener and calls a method to do something useful. Cool, right? So how does the screen know that you touched it?




Well, let’s look at a mathematical model as a starting point.

First of all, capacitance is a measure of an objects ability to store energy in an electric field. You know, like in a capacitor. 

The equation for this is C=\epsilon* (area of the capacitive plates) / (distance between the plates) (C=\epsilonA/d where /epsilon is the electromagnetic permittivity of the space between the points being examined.

Since the size of your screen doesn’t change and the permittivity is a constant, we can adjust the capacitance of our cellphone by pressing on the screen. This in turn reduces the distance between the plates our or theoretical capacitor (or sections of screen in reality) and increases the capacitance. Within an electrical circuit, this change in capacitance generates a tiny change in current that is read by the processor.

Since the human body is a fantastic conductor (when compared to wood), a finger pressing on the screen can conduct this current generated in the change in capacitance. By aligning the screen in a matrix (very similar to display pixels (1080p and such)) the processor is able to determine where the touch is occurring on the screen by assigning an x,y coordinate plane. As the finger moves, the x and y coordinates move, and the processor knows where the finger is located. This method is used in most every model of touch screen. Now, the part about wood, fruit, hot dogs…. An object needs to be conductive in order to operate the capacitive touch features of the screen. Since a hot dog is mostly salt water, it is HIGHLY conductive (slightly higher than your sweaty finger). Fruits contain a lot of sugars, which in and of themselves are not conductive. However, when dispersed in water, they can create dipole-dipole connections at the molecular level which allows a path for electron flow. This is less conductive than your fingers or tasty hot dogs. Lastly, wood is mostly made up of carbon bonds. These have no free electrons and make miserable conductors. Cotton shares similar properties. This is why your phone in your pocket results in no flow of current from the phone to the outside world. Therefore, there is no touch input received by the processor.

Hopefully this will help you the next time you decide to use a hotdog as a stylus. We’re not judging.

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