这篇文章介绍了一种名为「焦耳小偷」的简易电路，能够从看似耗尽的电池中榨取残余能量。当电池电压不足以驱动灯泡或LED时，电池并非真正「死亡」，其中仍含有化学能。透过变压器与电晶体的巧妙搭配，这个电路可以将低电压提升至足以点亮LED的水准，让电池的使用寿命大幅延长。

文章解释了焦耳小偷电路的核心原理：变压器利用法拉第感应定律，当一个线圈中的电流发生变化时，会在另一个线圈中产生感应电压。电晶体则充当电流的开关阀门，每秒数千次地开关电流，产生振荡效应，使变压器的次级线圈产生高于电池原始电压的电压尖峰，从而点亮需要3伏特的白光LED，即便电池只有1.5伏特。

焦耳小偷电路不仅是一个有趣的物理实验，更有广泛的实际应用。高端手电筒内建了微型化的焦耳小偷电路（即升压转换器），使其仅用单颗电池即可运作。太阳能板系统也使用类似的升压转换器，在阴天电压不足时仍能为家用电池充电，例如将10伏特的输出提升至可为12伏特电池充电的水准。

This article introduces the "joule thief" circuit, a simple device that extracts residual energy from seemingly dead batteries. A battery still contains chemical energy even when its voltage drops too low to power a device. By combining a transformer and a transistor, this circuit boosts the low voltage to a level sufficient to light an LED, dramatically extending battery life.

The circuit works based on Faraday's law of induction. A transformer with two coils wrapped around an iron core generates an induced voltage in the secondary coil whenever the magnetic field changes. A transistor acts as a rapid switch, toggling the current on and off thousands of times per second. This creates voltage spikes in the secondary coil that exceed the battery's original 1.5 volts, reaching the 3 volts needed to illuminate a white LED.

Beyond being a fascinating physics demonstration, the joule thief has practical real-world applications. High-end pocket flashlights incorporate miniaturized versions called boost converters, enabling them to run on a single battery. Solar panel systems also use boost converters to step up voltage on cloudy days, allowing panels producing only 10 volts to charge 12-volt home batteries, making this technology both widespread and essential.