You can make an electromagnet, as you’ve already seen, by passing dc through acoil wound around an iron rod. But there will still be a large, constant current in the coil.The coil will probably get more or less hot, as energy is dissipated in the resistance ofthe wire. The battery, too, or power supply components, will become warm or hot.If the voltage of the battery or power supply is increased, the wire in the coil, ironcore or not, will get hotter. Ultimately, if the supply can deliver the necessary current,the wire will melt.Coils and alternating currentSuppose you change the voltage source, connected across the coil, from dc to ac (Fig. 13-3). Imagine that you can vary the frequency of the ac, from a few hertz to hun-dreds of hertz, then kilohertz, then megahertz.232 Inductive reactance13-2A coil connected across asource of dc.13-3A coil connected across asource of ac.At first, the ac current will be high, just as it is with dc. But the coil has a certainamount of inductance, and it takes some time for current to establish itself in the coil.Depending on how many turns there are, and on whether the core is air or a ferromag-netic material, you’ll reach a point, as the ac frequency increases, when the coil startsto get “sluggish.” That is, the current won’t have time to get established in the coil before the polarity of the voltage reverses.