Portables and Magic Trackpad: Jumpy or erratic trackpad operation
Learn what to do if the cursor jumps or moves without the trackpad being touched. The cursor may also act strangely when the trackpad is touched.
The Apple Magic Trackpad and built-in trackpads on Apple portable computers work on a principle called coupling capacitance. As your finger moves over the surface, the trackpad evaluates the change in capacitance between two layers of measurement electrodes built into the surface of the trackpad and translates that to cursor movement.
Verify only one part of your finger is touching the pad. You cannot use a pen or other object--the trackpad is designed to work only with your finger. Also confirm that you are not resting your wrist on or very close to the pad. The trackpad may interpret this as your finger touching it and make the cursor move in that direction. If you suspect either of these is causing the cursor's behavior, try raising your wrist in the air and touch the pad with only the tip of your finger. If the symptom goes away then, you know that one of the above is the cause, and you should adjust the position of your wrist or finger.
If you have sweaty hands or if moisture collects on the pad, this may also confuse the trackpad. Wiping off the trackpad with a cloth or tissue usually fixes this issue. If the issue goes away when a piece of paper is put between the finger and trackpad you may want to consider purchasing a Teflon applique to fit over the pad to prevent the moisture from having direct contact with the trackpad. Before installing the Teflon applique, confirm the trackpad is clean and dry. You can use a mild glass cleaner sprayed onto the cloth, not onto the trackpad itself to clean the trackpad.
Oil or lotion can also cause the same issue. Do not use hand lotion, or consider purchasing a Teflon pad to protect the trackpad. Before installing the Teflon pad, confirm the trackpad is clean and dry.
Jewelry may also cause interference. Try removing rings or bracelets and see whether the jumpy or erratic behavior disappears.
Apple has had some reports of this behavior related to power supplies made by a third party company. If you are using one of these power adapters, unplug it and try running from the Apple battery to see how the trackpad behaves. Also try using the Apple AC adapter instead of the third-party AC adapter. If the trackpad behaves normally while running from the Apple battery or adapter, there is most likely an issue with your third-party adapter.