In electronics, undesirable low frequency currents interfering with a desired signal, and usually caused by a poorly screened alternating mains supply.
在电子学领域，一种不希望发生的低频电流，它干扰所要求的信号。通常这种现象是由于交流供电线路屏蔽不良引起的。
My new house is not yet connected to the mains.
我的新房子还没接上水、 电、 煤气呢.
I turned the gas off at the mains before I went out.
我出门之前，把煤气的总阀关掉了。
