Last time I talked about one of the dozens upon dozens of uses of induction loops. Normally we just refer to these devices as inductors. By definition an inductor is a loop of wire. These wire loops are very, very similar to electromagnets in design. The only difference is that in an electromagnet, we energize the coil with a direct current and hold it steady. This creates a magnetic field around the coil, which is typically concentrated by an iron core. Electromagnets can also be driven by alternating current, but this will result in the pole of the electromagnet oscillating from north to south at roughly the frequency of the alternating current. We typically don’t create AC electromagnets because the changing current gets resisted by the magnetic fields resulting in hysteresis losses. Though with different materials, those losses can be mitigated.
When direct current is initially applied to an electromagnet, there is a small amount of initial “resistance” that is essentially the result of the electromagnet acting as an inductor. The applied current isn’t instantaneous and the changing current will be resisted (this is the electromagnet acting as an inductor). This quirk does not cause any design difficulties because the direct current eventually levels off and the electromagnet coil becomes just an ohmic resistor. However, when shutting the electromagnet down, the magnetic field collapses. This collapsing magnetic field will induce a current in the coil which can cause sparking across any switches present in the circuit. To prevent this, blocking diodes are used to redirect induced current back into the coil where it can be safely dissipated as ohmic losses.
A very specific kind of inductor shows up in radio receivers. This inductor forms what is called an LC circuit (L = inductor, C = capacitor). An LC circuit is also known as a resonant circuit or tuned circuit. Radio waves are a form of electromagnetic radiation. This also includes, in descending order of frequency, gamma rays, x-rays, UV, the visible light spectrum, infrared, microwaves, FM radio waves, AM radio waves, and finally, long radio waves. Electromagnetic radiation itself is composed of photons. As we know, electrical and magnetic fields propagate away from their sources into space. This property is what we use to transmit information wirelessly via radio. A radio in its most basic form consists of an antenna that is acted upon by the electromagnetic radiation, a tuning circuit and some kind of amplifier. This induces a tiny, tiny current in the antenna which is then amplified to produce an audible signal via simple circuitry. The LC circuit comes into play because an antenna, which is just a wire, cannot discern between all the different radio frequencies that are acting upon it. The LC circuit resonates only at a specific frequency. Other frequencies are discarded into ground, essentially blocking them from reaching the rest of the circuitry.
Interestingly, because electromagnetic radiation is a form of energy, a radio need not necessarily supply its own power to function. The energy in the electromagnetic waves themselves power the radio. These radios are called crystal radios. When they were first being discovered, they used a semiconducting crystal as a rectifier (a component that only allows charge to move in one direction). Today we use germanium diodes as rectifiers. These radios can be very difficult to construct successfully, but are very rewarding when you do! As a kid, I built one where I used a metal window frame as an antenna.
Regardless, radios are able to function because of the amazing properties of inductors. From this simple concept we get every single piece of wireless communication we use today, from WiFi to microwave cell signals to ELF (Extremely Low Frequency) waves used to communicate with submerged submarines. It’s amazing how something so simple can be used for so many different purposes.