The Knowledge: How to Rebuild Our World From Scratch (24 page)

Electricity is wonderful stuff: it will shoot virtually instantaneously along a wire laid for it and produce a noticeable effect far away from the operating switch—illuminating a light bulb in another room, for example. But to communicate between buildings, cities, or even continents, you can’t simply extend a light bulb–powering circuit and flash messages to one another. Energy-sapping resistance is your enemy here, as there won’t be enough voltage to power a bulb over any meaningful distance. A good electromagnet, built as we saw in Chapter 8, will generate an appreciable magnetic field from even a feeble current, though. Position a lightly balanced metal lever over the end and you can use it as an exquisitely sensitive switch, pulled closed to sound a buzzer whenever the electromagnet is energized. A relay-controlled buzzer at both ends of a long telegraph wire will allow remote operators to hear whenever either is sending current.

Messages can be sent one letter at a time by representing them as combinations of short or long squirts of current—dots and dashes. All you need to do first is agree with the guy at the other end of the telegraph cable how you will represent each letter of the alphabet, and then send your first post-apocalyptic e-mail down the wire. Exactly how you organize this doesn’t really matter, but with a little bit of forethought on how to ensure that the coding system is both rapid and
reliable, you’ll probably reinvent something similar to Morse code. In this system, the most commonly used letters in the English alphabet are represented by the simplest forms: E is a single dot, T is a single dash, A is dot-dash, and I is dot-dot.

Regularly spaced relay stations will boost the current along the next section of wire and so allow globe-spanning telegraphic communications. But the laying and maintenance of wires draped across the continents and ocean floors is difficult. So is there a better way? Can you communicate using electricity, but without the bothersome wires needed to carry current?

Let’s look more closely at the yin-yang relationship of electricity and magnetism. If a changing electric field can generate a magnetic field, and a changing magnetic field can in turn induce an electric field, then you ought to be able to create a ripple of mutually supporting energies. Indeed, such electromagnetic waves propagate even through a perfect vacuum with no matter present to carry the disturbance (unlike a sound or water wave): electricity and magnetism combine to travel like ghosts through the universe.

The golden sunlight streaming through my window is itself nothing more than a welding of electric and magnetic fields. Indeed, everything from X-ray machines, ultraviolet tanning beds, infrared night-vision cameras, and microwave ovens to radar, radio and TV broadcasts, and the ultimate expression of modern life, this free Wi-Fi hotspot I’ve hopped onto with my laptop, are all based on varying forms of light. The electromagnetic spectrum is a broad swath of waves with different frequencies of vibration of the coupled electric and magnetic fields, stretching from dangerously energetic gamma radiation to long-wave radio, but all propagating at the speed of light.

But it is radio waves that interest us here. Not only are they relatively simple to make and catch, but they can also be imprinted with information to carry over vast distances. It is this radio transmitter and
receiver technology that you’d ideally want to recover as a means for long-range communication during the reboot.

Let’s start with the slightly easier task of building a radio receiver. Dangle a long piece of wire from a tree, with the bottom end stripped of any insulation and buried in the earth to ground it. This is your aerial, and the rapidly fluctuating electromagnetic fields of any passing radio waves will drive electrons in the metal to slosh up and down the wire—an induced alternating current. But in order to drive a pair of earphones to hear anything, you need some way of keeping either the negative or positive parts of the wave and discarding the other half.

Any material that allows electricity to flow through it in only one direction, blocking the reverse tide, will accomplish this, “rectifying” an alternating current into a series of pulses of direct current. Luckily, many different crystals turn out to exhibit this marvelously useful property. Iron disulfide, also known as fool’s gold for its deceptive appearance, works well and is easy to spot. Another mineral, galena (lead sulfide), is also used commonly in crystal radio sets. Galena is the main ore of lead, found in large deposits all over the world, and has been mined throughout history to produce plumbing pipes, church roofs, musket shot, and rechargeable lead-acid batteries.

Connect the crystal into your aerial-earphone circuit by placing it in a metal holder, and make a second contact with it using thin wire, known as a cat’s whisker. Rectification happens at the connection between the crystal and the point of contact, but the effect is elusive, and finding a sweet spot by trial and error requires a lot of patience. Nonetheless, even in the absence of any human broadcasts, this rudimentary setup may allow you to pick up the radio emissions from natural sources like lightning storms. In fact, a rudimentary radio transmitter—the spark gap generator—works by creating a rapid series of artificial lightning discharges.

Spark gap generators leave a small gap in a high-voltage electric circuit so that a spark repeatedly leaps across it. Each spark releases a
surge of electrons along the aerial and the emission of a brief burst of radio waves. If the transmitter circuit sparks thousands of times a second, releasing a rapid train of radio pulses, a buzzing tone will be heard in the earphones of receiver sets. Insert a switch on the low-voltage side of the transformer that powers the spark gap to control when the circuit is energized and transmits radio waves, and again encode your message in dots and dashes.

Ideally, though, you want to be able to transmit sound over the radio waves, allowing conversations between individual radio operators or the broadcast of news to a widely spread audience. Morse code involves crudely turning the radio waves completely on or off, but conveying sound requires a more refined manipulation, known as modulation of the carrier wave. The simplest scheme is called amplitude modulation (AM), whereby the intensity of the carrier wave is varied more smoothly between these two extremes: the gentle contours of the sound wave are imprinted on top of the frenetic fluctuations of the radio wave. Thankfully, the cat’s-whisker crystal detector also works admirably to “demodulate” the signal in the receiver. The one-way-street behavior of the crystal junction coupled with the smoothing effect of a capacitor strips away the high-frequency carrier wave, leaving behind the broadcaster’s voice or music.

Unless you have only a single high-powered transmitter nearby, the signal you hear with this bare-basics radio receiver will be a confused mash-up of stations: the aerial picks up a variety of transmissions on different frequencies of carrier wave and passes them all on to your earphones. Adding a few extra components to your electronic machines will allow you to tune these radio sets. Tuning makes a radio transmitter more efficient by packing the broadcast energy into a narrow span of radio frequencies, and a tuned receiver plucks only the transmission frequency you are interested in out of the jumbled cacophony of the radio spectrum.

As we’ve seen, a radio wave is fundamentally an oscillation, and the
magnetic and electric fields composing it alternate with a particular rhythm or frequency just like the swinging pendulum of a clock. So to tune a radio transmitter or receiver, you need to include a circuit that electrically oscillates with a particular rhythm and resists other closely matching frequencies. You need to harness the power of resonance.

Think of it this way. A child on a swing will oscillate back and forth with a particular frequency, just like any pendulum. If you deliver a series of tiny pushes at the right moments the child will swing higher and higher. But pushing with a different rhythm from this resonant frequency will get you nowhere.

Building a basic oscillator circuit that beats with a fixed rhythm uses a gratifyingly elegant combination of a capacitor and an inductor. A capacitor is made up of two metal plates facing each other, sandwiching a layer of insulation between. Any voltage across the device herds electrons onto one of the plates until it becomes so negatively charged it resists further filling. A capacitor serves as a reservoir of electric charge and can release this in a sudden torrent, such as in the flashbulb of a camera. An inductor coil is essentially an electromagnet, but the effect of an inductor is far more than just attracting metal objects. While resistance resists the flow of current, inductance resists any change in the flow of current. So the capacitor and the inductor both serve as refillable stores of electrical energy: the capacitor in the form of an electric field between its facing metal plates, the inductor as a magnetic field surrounding the coil. Wire these two components opposite each other and the simple loop circuit miraculously comes to life.

As the electron-laden capacitor plate dumps its stored charge, it pushes a current around the circuit and through the inductor to create a magnetic field, until the capacitor plates have equalized. Now the magnetic field around the inductor begins to collapse, but as it does, the shrinking field lines sweep over the coil to induce a current in the
wire (the generator effect), and continue to pump electrons to the other capacitor plate—amazingly, the collapsing magnetic field is able to temporarily sustain the very electric current that created it in the first place. By the time the inductor field has diminished back to nothing, the opposite plate of the capacitor has become fully charged, and now pushes the current back in the opposite direction, flowing through the coil again.

The energy flows back and forth between capacitor and inductor in this way, being repeatedly interconverted between electric and magnetic fields, like a pendulum swinging to and fro thousands of times every second—at radio frequencies.

The beauty of this disarmingly simple oscillating circuit is that it wants to tick at only its own natural frequency, and will resist other frequencies. You can change the resonant frequency of this circuit, and so retune your transmitter or receiver, by changing the properties of one of the two components. The capacitor is the easier to adjust: rotating D-shaped metal plates past each other varies their overlap and so the charge that can be stored. The tuning knob on old radio sets was therefore often connected to a variable capacitor in the oscillating circuit. Modern transmitters and receivers can be tuned so finely that the radio spectrum has been thinly sliced like a ham on the delicatessen counter and shared between myriad applications: commercial radio and TV stations, GPS signals, emergency service communications, air traffic control, cell phones, short-range Wi-Fi and Bluetooth, radio-controlled toys, and so on. Indeed, spark gap transmitters are now illegal, as they are such unrefined sources, leaking emission in fat smears across the radio spectrum, that they essentially spam broad regions of neighboring radio bands.

The other crucial elements for audio broadcasts are of course a microphone, to convert sound waves into voltage variations in the transmitter circuit, and earphones or speakers to transform the received
electric signals back into sound. In fact, microphones and earphones are basically the same device. Both contain a diaphragm that is free to vibrate to either create or respond to sound waves, fixed to a coil of wire that then moves over a magnet, and so they harness the same reversible electromagnetic effects as motors and generators.

A more sensitive version can be built using a piezoelectric crystal, which has the curious property of generating an electric voltage when it is flexed. Such a sensitive crystal earphone is needed to hear the vanishingly faint output from a cat’s-whisker radio detector. Potassium sodium tartrate (or “Rochelle salt,” after the hometown of the seventeenth-century apothecary who first created it) works nicely in this respect. This salt can be prepared by mixing hot solutions of sodium carbonate and potassium bitartrate (widely known as cream of tartar), which can be gathered as the crystals that form inside wine fermentation casks.

We can be confident that a rebooting civilization could quickly reattain radio communications from absolute basics, even without deriving the complex electromagnetic equations or having the capability to manufacture precision electronic components. It’s already been done in our own recent history.

During the Second World War, both soldiers holed up at the front lines and those imprisoned in POW camps built their own makeshift radio receivers for music or news of the war effort. These ingenious constructions reveal the sheer variety of scavenged materials that can be jury-rigged to create a working radio. Aerial wires were slung over trees, or disguised as clotheslines, and sometimes even barbed wire fences were appropriated for the task. A good grounding was achieved by connecting to cold-water pipes in the POW barracks. Inductors were constructed by winding coils around cardboard toilet rolls, the scavenged bare wire insulated by candle wax, or in Japanese POW camps by applying a paste of palm oil and flour. Capacitors for the
tuning circuit were improvised out of layers of tinfoil or cigarette-pack lining, alternating with newspaper sheets for insulation; the wide, flat device was then curled like a jelly roll to make a more compact component.

The earphone is a trickier component to improvise, and so was often salvaged from wrecked vehicles. Rudimentary alternatives were constructed by coiling wire around a core of iron nails, sticking a magnet on the end, and lightly positioning a tin can lid over the coil to vibrate weakly with the received signal.

Perhaps the most ingenious improvisation of all, however, was in creating the all-important rectifier, needed to demodulate the audio signal from the carrier wave. Mineral crystals like iron pyrite or galena were unobtainable on the battlefield, but rusty razor blades and corroded copper pennies were discovered to serve just as well. The blade or coin was fixed to a scrap piece of wood alongside a safety pin bent upright. A sharpened pencil graphite was firmly attached to the point of the safety pin (often by winding spare wire tightly around the two), and the springiness of the arm functioned adequately as a cat’s whisker, allowing fine readjustment of the pencil graphite across the metal oxide surface until a working rectifying junction was found.