Batteries that no longer work in a device still contain chemical energy and measurable voltage, even though the current they produce is too weak to power lights or other loads. A simple electrical circuit combining a transformer and transistor can tap that residual energy, a technique known informally as a "joule thief" circuit.
The method demonstrates Faraday's law of induction, the same principle used in electric generators and induction cooking stoves. In a basic circuit, electric current from a battery flows through a light bulb's filament, heating it to approximately 4,500 degrees Fahrenheit and producing light. As a battery's voltage drops during use, it generates progressively less current until it can no longer produce visible light.
Most modern devices use light-emitting diodes instead of incandescent bulbs because LEDs convert electrical current directly to light rather than relying on heat, making them far more efficient. A white LED typically requires 3 volts to operate, compared to 1.5 volts for an incandescent bulb powered by a single battery.
