How do batteryless flashlights work and can the technology be expanded to other devices?
The main idea behind batteryless flashlights is to use the physical motion of the flashlight itself to provide the energy to light up the bulb -- that is, you have to shake it. What happens is always a variant on the following basic idea:
If you have a coil of wire and drop a magnet through it, this will make some electricity flow through the coil and whatever it's attached to. This electricity can be stored to power a light bulb. More shaking means more energy to store up and a longer time for the bulb to stay lit. If you run out of energy, you just shake the flashlight to make the magnet go in and out of the coil. (This basic principle of "moving magnet plus coil makes electricity" is the foundation for electrical generators, so in a sense this is old technology).
The component that stores the electricity is usually a capacitor, which is basically just a couple of sheets of metal held very close together, but not touching. This is also old technology in a sense, but it's only recently that manufacturers could make good enough, small enough capacitors to make the batteryless flashlight concept work. Some "batteryless" flashlights cheat and use a little rechargeable battery instead of a capacitor, in which case they're only batteryless in the sense that you need not, and cannot, change the batteries!
Avoiding the use of batteries has a lot of nice environmental advantages since most batteries have metals in them that are not at all nice to toss into the trash.
Why don't we just make everything like this? The catch is in the power requirements. Your eyes are quite sensitive and it doesn't take a lot of electricity to make enough light to see well by. Powering up a drill or a laptop is another story and you would find the physical work required to get these things to run would be far more than you would be willing to do -- so this sort of batteryless technology is really limited to low-power devices.
Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819. 
