Archive for July 14th, 2010

Science Wednesday: Hold the phone; or, conduction function

by Toby J. Swaford, K-12 Education Coordinator

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When I saw the news about Steve Jobs’ reported response to what many may consider a design flaw in the new iPhone 4, I was immediately transported to those golden days of Vaudeville.

“Oh, Mr. Jobs!”

“Yes, Mr. Customer?”

“My iPhone doesn’t work when I hold it like this.”

“Then don’t hold it like that.”  Ba-dum-da!

Bad jokes aside, there’s a reason that Apple’s new phone may occasionally not pick up a mobile signal as well as it should. Strangely enough, it’s also the same reason the iPhone and many other touch screen devices work the way that they do – conductivity.

Electronic devices can use several methods to detect a person’s input on a touch screen. Most involve the use of circuitry that sense changes in a particular state. Some look for changes in the reflection of waves, including sound waves and beams of near-infrared light. Transducers detect vibrations caused when your finger hits the screen’s surface, while camera based systems monitor changes in light and shadow. Many touch screens, including those on the iPhone, respond to changes in electrical current.

Capacitive touch-screens use a layer of material to hold an electrical charge; touching the screen changes the level of the charge at the point of contact. Resistive screens depend on the pressure from your finger to cause the conductive and resistive layers of circuitry to touch each other, changing the circuits’ level of resistance at that particular point. By detecting these changes the screen communicates where your finger is placed to the phone’s processor and software, transforming that simple touch into a series of commands. The fact that all of this takes place almost instantaneously is pretty impressive.

If you’re wearing something between your finger and the screen that keeps you from changing the level of conductivity (such as a pair of gloves), conductive screens have their shortcomings. Some do-it-yourselfers have come up with a quick solution to that particular wintertime problem and have sewn conductive wire into the fingertips of their gloves. The wire serves the purpose of completing the contact between the operator’s skin and the device’s touch screen. Other creative solutions have ranged from using an exposed nose or chin to activate the screen, to using a sausage link as a stylus for manipulating the on-screen controls.

So, how does the conductivity of human skin (and breakfast meats) cause the new iPhone to suddenly drop calls or lose its mobile signal? It has to do with the antenna built into the device. The design incorporated into the iPhone 4 is a bit different from other mobile phones because its antenna is built into the outer case of the phone, not inside the device itself. This brings the hand holding the phone into direct contact with the antenna.

This doesn’t block the signal as much as it changes the length of the antenna, with your body’s own conductivity making you a part of the antenna system. If you’ve ever held a set of old-fashioned “rabbit ears” while standing on one foot, you may have some idea as to what I’m talking about. Unfortunately in this case, the mobile phone is looking for a very specific bandwidth and your body changes the range that the antenna is able to find, causing significant loss of signal.

If you manage to touch both sets of antenna at the same time, you may accidentally connect them together. As your skin, usually covered with a bit of moisture, salts, and oils, comes in contact with two pieces of metal your natural conductivity causes electrons to flow from one piece of metal, through your body, and into the other metal surface. This flow of electrons can be detected using a device such as a microammeter, and is the basis of a popular exhibit in the lobby of the Fort Collins Museum & Discovery Science Center. Incidentally, the chemical reaction between metal objects and the oils on your skin is also what gives things like coins and keys their distinctive “metallic” smell.

The solution is simple enough; you could, as Mr. Jobs suggests, adjust the way in which you hold the phone. If you choose not to go against millions of years of evolution and continue to use your opposable thumb, an alternative would be to create a nonconductive barrier between your skin and the edge of the device. Bumpers, rubber covers designed to protect the edge of the iPhone are being made available to help with this situation; although official Apple Bumpers may set you back almost thirty bucks. A low-tech and cost effective answer involves simply placing a piece of insulated electrician’s tape over the antenna.

July 2010

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