After Apple launched the iPhone, its smooth capacitive touch screen has become increasingly popular in the market. The capacitive screen has a longer life and lower power consumption than the resistive screen. With the mass production and technical improvement of capacitive screen, capacitive screen has replaced resistive screen as the mainstream touch screen. Maybe many people will wonder why the mobile phone knows where I touch when my fingers slide on the screen glass?

In fact, the glass on the mobile phone is not an ordinary piece of glass. The screen of the mobile phone includes a touch detection device and a touch controller. The touch detection device is used to detect the user's touch position, while the touch controller converts the collected touch information into contact coordinates that can be read by the CPU, and finally the CPU executes the touch information.
When the hand touches the screen surface, the finger and the touch surface form a coupling capacitance. Since the touch surface is connected with high-frequency signals, a certain amount of charge will be transferred to the human body (it is safe to be electrically immortal). In order to recover the loss of these charges, the charge will be added back from the four corners of the screen. The amount of added charge is proportional to the distance of the touch, We can then calculate the position of the touch point.

Touch the screen with your hand and there will be response. Do other materials also have response? It also said that it must be able to form a coupling capacitor with the touch screen and take the current away from the screen, so this thing must be a conductor, and it must have a certain contact area.

In the following video, we opened a drawing application in the iPad. If the tool we used to touch the screen responds, it will definitely leave traces in the drawing application.

In the first video, we used a metal spoon. We can see that holding the spoon screen directly with our hands is responsive, but if we hold the spoon through an insulator, the touch screen will not respond. We also tried other sharp objects with small contact area with the screen. Although they are conductors, the screen does not respond.

In the second video, we used a metal plate with a relatively large contact area with the screen. To verify whether the metal plate reacts to the touch screen, we used an insulated rubber to move the metal plate, and the screen also reacts.

The third one we tested was fruit. We did not touch the fruit with our hands, and then shook the apple on the screen. The screen also responded.

The fourth touch pen we tested is specially designed for capacitive screen. It can be used when touched by hand at ordinary times, but it can no longer be used when we isolate it with insulating rubber.