If you have a cell phone or a pager, then you know that having it ring in the middle of a movie or performance is enough to get you killed in some cities. Vibrating devices that quietly replace the ringer are therefore life-saving devices that are an important part of urban survival!
There is, however, a device that takes vibration to high-tech extremes. Any parent whose child owns a Tickle-Me-Elmo doll has experienced this technology. Elmo has a vibration system (designed to simulate body-shaking laughter) that is powerful enough to cause many children to drop the toy. The vibration system inside a pager works exactly the same way on a smaller scale, so let's use Elmo as an example.
Here is a Tickle-Me-Elmo doll:
Minor surgery reveals the control unit:
Inside the control unit (on the right hand side in the above image) is a small DC motor which drives this gear:
You can see that, attached to the gear, there is a small weight. This weight is about the size of a stack of 5 U.S. nickels, and it is mounted off-center on the gear. When the motor spins the gear/weight combination (at 100 to 150 RPM), the off-center mounting causes a strong vibration. Inside a cell phone or pager there is the same sort of mechanism in a much smaller version.