The complement to a capacitor is an inductor, which is merely a coil of wire, frequently wound on an iron core. DC has no trouble passing through an inductor since it is, after all, made of wire. But alternating currents have an increasingly difficult time, and as the frequency is raised less and less current will appear at the other end.

    Inductors are measured in Henries, though again, most of the ones you're likely to encounter will be rated in millihenries or even microhenries. As more and more wire is added to the coil, the larger its value will be, and the less able high frequencies are to get through it. The amount of iron used for the core also has an effect, with more iron increasing the inductance.

    In practice, inductors are not always used where they could be, since they may be simulated less expensively using other components. (A complex issue not worth getting into here.) Copper wire and iron are quite expensive these days, plus an inductor's bulk and weight don't lend themselves to today's miniaturized equipment. However, the real point is that an inductor is also frequency sensitive, and it is exactly the opposite of a capacitor.