For AC power, there is no distinction between positive and negative poles. When using a multimeter in AC mode to measure, the results are actually the same for both positive and negative sides. However, when using a multimeter in DC voltage mode to measure DC power, there is actually a "-" sign difference between the positive and negative sides. This can also distinguish between AC and DC, but there is still a problem. In order to use a multimeter to measure this result, it is necessary to know whether the power source is DC or AC, otherwise selecting the wrong mode will not display the result.