家庭和工厂所用的电大部分是交流电。交流电的优点,主要是:它可以經过变压器,很方便地把电压升高或者降低,进行長距离的輸送,使中途損耗的电力減到很少。但是在日常生活和某些工業部门中,有些地方还需要使用直流电(圖1)。例如,無線电、电话以及化学工業中的电解、电鍍、电冶等等,都必须使用直流电。因此,尽管發电厂中發出来的是交流电,輸送給我們的也是交流电,我们有时候还需要把交流电改变成为直流电,这便叫做 Most of the electricity used by homes and factories is AC. The main advantages of AC power are that it can be passed through a transformer and it is convenient to raise or lower the voltage for long-distance transmission, so that the amount of power lost in the middle is reduced to a minimum. However, in everyday life and certain industrial sectors, DC power is still needed in some places (Figure 1). For example, electrolysis, electroplating, electrowinning, and the like in radio, telephone, and chemical industries all must use direct current. Therefore, even though the power plant sends out AC power and the AC power is delivered to us, we sometimes need to change AC power to DC power. This is called