Touchscreen trouble? It could be zombie finger

Here's why capacitive screens don't respond to every touch

Published: June 02, 2015 11:30 AM

Some smartphone and tablet users are afflicted with a malady. No matter how hard they press on the display, they just can’t seem to get the device to acknowledge their touch. These people may have the same problem with laptop touchpads. In layman’s terms, they suffer from zombie finger.

“The capacitive touch sensor is—to most people—this kind of magical thing,” says Andrew Hsu, Ph.D., a pioneer in touchscreen tech at Synaptics, a major supplier of the technology to electronics manufacturers. “In an ideal situation, you barely touch the surface of the screen and the sensor is able to detect the presence of your finger.” In some cases, however, that finger confounds the technology.

“It’s a problem we’ve been wrestling with for 20 years now,” says Hsu. “It’s a very delicate balance. We spend a lot of time essentially trying to determine whether a user has touched the surface or not.”

To understand why one finger gets noticed while another is ignored, you need to know how a capacitive touchscreen works. Unlike the resistive screens, which rely on mechanical pressure to register each touch, a smartphone or laptop touchpad generates a small electric field. In fact, you don’t even have to make contact with the touchpad for the sensor to detect your finger. Because the human body conducts electricity, a fingertip in close proximity to the glass will absorb the electrical charge and create a measurable disturbance in the field, alerting a grid of electrodes on the screen and enabling the phone to register the command.

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To satisfy consumers, capacitive touchscreens must be nimble enough to recognize the dainty finger of a toddler, the bony digit of a an elderly person, and the meaty stab of a sumo wrester. What’s more, software algorithms need to filter out the “noise” generated by grease and grime on the glass, not to mention the overlapping electrical fields generated by fluorescent lights, poorly designed charging stations, even other components inside the device. “It’s one of the reasons why the mobile phone has more processing power than the computers used to send a man to the moon,” says Hsu.

All things considered, capacitive touchscreens offer clear advantages for cell phones and tablets. They stand up well to the wear-and-tear of constant use, they don’t detract from picture quality, and they permit multitouch gestures. And for the record: Despite what you might have heard, they perform fine whether you’ve got a hot or cold hand.

In the end, though, capacitive touchscreens are not foolproof. Living, breathing people with thick callouses on their fingers—think guitar players or carpenters—struggle with these touchscreens because the dead skin on their fingertips prevents the flow of electricity. People wearing gloves tend to experience trouble. People with very dry hands, too. “I’ve also heard of women with really long fingernails having problems,” says Daniel Tower, an engineer at Wacom, which makes drawing tablets and styluses. Basically, anything that limits your hand’s conductivity is a potential pitfall.

So what should you do if you have zombie fingers? You might try licking your fingertip or, better yet, applying a water-based moisturizer to your hands. And, if you can’t bear to give up playing electric guitar or having designer nails, think about using a touchscreen stylus to funnel the electricity into your mitts.

Don’t have one handy? People in South Korea have discovered that a pork link will do the job. “There’s moisture in that sausage,” says Hsu. “So long as your body is in contact with it, it has enough conductivity to affect the electric field.”

Of course, the Slim Jim approach presents other challenges. “That only works if you’re not hungry,” says Tower’s Wacom colleague Doug Little.

—Chris Raymond

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