Engineers at Perpetuum Ltd. in Britain ( designed the PMG7, an energy-harvesting generator, with wireless, battery-free sensors in mind. The small generators convert kinetic energy from equipment vibrating at 50 or 60 Hz into electricity. One such device generates up to 5 mW, enough to power a wireless transmitter sending 6 kbytes of data every few minutes. It could also power a temperature sensor sending smaller amounts of data but several times per second. The generator will work in most industrial settings and needs minimal vibration levels of 25 mg. (A mg is a thousandth of a g.) The device can be screwed in place or held by magnets. The manufacturer says the unit needs no maintenance.