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From Issue #9 January 31, 2013

His Bright Materials

Seemingly new technology has deep roots in the past: electroluminescent materials and electrets.

By Daniel Rutter Twitter icon 

EL night lights have been common since the 1960s, with some still in use after 50 years of continuous operation. Photo by Marco Arment.

The Romans had flush toilets and central heating, and they used concrete in their buildings. Nitrous oxide was discovered in 1772, though nobody figured out that it worked as an anesthetic (as opposed to a recreational drug!) until 1844. And Antonie van Leeuwenhoek invented a microscope good enough to see individual cells in the 1660s.1

Most people — most of the rather odd people I hang around with, at any rate — keep a few of these seemingly anachronistically early uses of technologies in their mental factoid warehouse. These bits help us explain unexpected relics, like the Roman aqueducts and buildings (like the Pantheon) that are still standing after well over a millennium and a half.

Many older bits of tech came along at the wrong time to become popular or were beaten to the market by seemingly superior alternatives.

Everything old is new again

These examples aren’t anomalies; rather, “old” technologies are lurking everywhere, and are often cousins to better-known tech. Take, for instance, two of the modern world’s little techno-miracles: high-brightness light-emitting diodes (LEDs), and super-strong neodymium iron boron (NIB) magnets.

LEDs work because of direct application of the principles of quantum physics, which is kind of amazing all by itself before you even notice that 100-packs of the things are selling on eBay for three dollars. The spookily strong field of NIB magnets (also known as just “neodymium” or “rare-earth” magnets) is similarly astonishing. A NIB magnet can manage 10,000 gauss at its surface, which is 10 times the peak strength of a common ferrite magnet. And, once again, the darn things are now so cheap as to be almost free.

High-intensity LEDs have been around for about 30 years, though they weren’t affordable until the 1990s. Rare-earth magnets were invented about 20 years ago. A couple of their technological cousins, though, are every bit as peculiar as LEDs and high-powered permanent magnets, but much older. They are the light-emitting capacitor and the permanent electret.

The Romans didn’t invent either of these things, but as far as the kids these days are concerned, they might as well have.

Glow, little capacitor, glow!

Light-emitting capacitors are better known as electroluminescent, or EL, materials. As is normal for capacitors, they’re composed of two conductive plates separated by an insulating “dielectric,” and as is compulsory for capacitors, they block direct current but allow alternating current to pass.

The thing that actually glows in an EL object is a layer of semiconducting phosphor — most often zinc sulfide plus a little manganese — deposited on the transparent dielectric. One or both of the plates are also transparent, so the light can get out. Plates are usually made of transparent conductive thin film, a less sophisticated version of the thin films used in LCD (liquid crystal display) monitors and TVs.

EL materials and LEDs were both discovered, in uselessly dim versions, in the surprisingly early 20th century. Practical EL materials were developed in the 1950s, and EL lights were illuminating Chrysler Imperial dashboards in 1960. The Apollo missions’ spacecraft also sported an electroluminescent display on their guidance computers.

There are some reasons why electroluminescence hasn’t become more widely used over the decades. It very efficiently converts electricity into light, for instance, but it never produces terribly much light. You can use it as a nightlight or a backlight, but not as a flashlight.

EL materials also have a lousy lifespan. The second you turn an EL light on, its phosphor starts slowly deteriorating and its light output starts falling. EL lamps seldom completely die — there are EL nightlights from the 1960s that still work today — but they’re a lot less bright after a few hundred hours of run time than when they were new.

Electroluminescence also, like an LED, gives you only one very narrow bandwidth of light — color, to its friends. You can mix phosphors to get something vaguely approximating white, but that’s bad for efficiency and makes the already dim light even dimmer. Only now are we starting to develop different EL colors with similar brightness.

(White LEDs do this too — they’re actually blue LEDs, and that yellow stuff you can see in there when the LED is off is a phosphor layer that converts some of the blue to other colors, summing up as white.)

That all contributes to why LEDs beat out electroluminescence in the lighting world. But that doesn’t mean our old friend never made it to market. In fact, it’s cheap and readily available, and can be fun to work with in ways that LEDs can’t be used. EL tape is now a discount-store item, and EL wire can be had for pennies. For a couple of bucks on eBay, you can get a meter of EL wire that draws current from two AA batteries for a surprisingly long time — enough EL wire to deck out your whole Tron costume won’t cost you a great deal more.

(Super-cheap EL wire can be a little less bright; both of its “plates” are actually just decidedly non-transparent copper wire, and the outer one just spirals around the dielectric.)

EL material is known by a variety of other names. Timex calls the EL material on its watch faces “Indiglo.” Cheap EL kits also often have “neon” in the title, because they do rather look like fluorescent lamps, though they operate on an entirely different principle.

Recently, EL materials have begun to be widely used for backlights on watches and monochrome LCDs, and for decorations. A casual observer could be forgiven for thinking that EL materials and high-brightness LEDs were developed at about the same time.

In the near future we may have substantially brighter and longer-lived EL materials to play with, so you could have household lighting that’s just a big softly glowing sheet stuck to the ceiling. Or an actual full-color EL screen, with low power consumption and a full 180-degree viewing angle.

Electret boogaloo

Electric fields are intimately related to magnetic ones but much harder to directly detect. Something with a huge static charge on it will raise the hairs on your arm as you reach for it, but that’s about it for direct perception of electric fields. This is all part of what makes the electret seem so witchy to me.

(The jury is out regarding whether walking under high-tension power lines at night while holding a fluorescent tube, and watching the tube turn into a lightsaber, counts as “direct perception.”)

Artist Richard Box’s 2004 Field installation: fluorescent light bulbs glowing from the electric field surrounding the overhead high-voltage power lines. Photo by Grete Howard.

You’ve probably seen electret printed on microphones and not thought much of it. But the electret that allows those mics to work is the electric-field equivalent of a permanent magnet. It has a permanent, fixed electric field around it. The electret in the cheapest such microphone ever made will probably retain its field for centuries.

Electrets aren’t even very hard to make; one manufacturing technique goes about it in the same way magnets are made. Just melt something made up of polar molecules — wax, for instance — and allow it to resolidify in a strong electrostatic field, and an electret will be the result.

In 1962, though, someone discovered that if you make an electret from PTFE (polytetrafluoroethylene, or “Teflon”) in the form of a thin metalized foil, and then attach that foil to one of your signal wires, and connect a close-set fixed metal plate behind the foil to the other, then vibration of the foil relative to the backplate will create a tiny AC voltage. This feeble signal must then be amplified to a more useful level by a powered transistor built into the mic; this is why electret microphones need a battery, or some other low-amperage source of a few volts.

Electrets don’t seem as new and exciting as EL materials, because crummy electret microphones abound in the “questionable hi-fi equipment” section of charity shops the world over. The electret microphone was invented in the ’60s, but the word “electret” was actually coined by Oliver Heaviside, he of the “Heaviside layer,” in 1885. And Heaviside wasn’t the first guy to experiment with them.

You don’t even have to deliberately make a magnet or an electret to experiment with them; weak magnets (“lodestones”) and electrets exist in nature. Quartz crystals, for instance, are faint natural electrets — though I recommend you keep this under your hat, in case it encourages any nearby crystal mystics.

Oh, and if you move a strong electret near a strong magnet, they’ll push each other around, but only when they’re moving relative to each other. And the force on the electret will be 90 degrees off from the direction you’d expect it to point if the electret were a magnet.

Everyday miracles

I don’t think the oldness and cheapness of electroluminescence-based gear and electrets makes them any less fascinating. If anything, it’s a bonus; anyone can experiment with this stuff today without spending more than pocket change. Get some rare-earth magnets and LEDs from your favorite online flea market, and marvel at the strangeness and magic of even the mundane things in the world in which we live.

I think everyone should assemble a little play-set of the building blocks of technology. Dolly Parton says she doesn’t mind being called a dumb blonde, since she knows she is neither of those things. And incomprehensible modern technology is often actually quite comprehensible, and not necessarily even modern.


  1. Van Leeuwenhoek monopolized the market for the tiny glass-bead lenses his microscopes used by fooling everyone into thinking he made them like any other lens — by laborious grinding. Actually, all he did was melt the end of a glass rod, keeping the blobs that turned out nice and round and re-melting the others! 

Dan Rutter is an Australian journalist who writes about science, technology, video games, computer hardware, gadgets, strange tales, and anything else that catches his attention. His websites are Dan's Data and How To Spot A Psychopath.

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