When Harry Potter first opens Tom Riddle’s diary, messages appear on the blank pages as if from nowhere. They fade in then fade out, leaving no trace that they had ever been written. This magical text act startles Harry.
If he had returned to London and waited a few years, though, he would have seen the Amazon Kindle in regular Muggle bookshops. Like Tom’s diary, this e-reader, and ones like it, leaves absolutely no mark when it shifts to a new page. It is magical, even without a powerful wizard’s presence within.
Disappearing text isn’t a new idea, either. Back in the 1950s, an electrician invented the Etch-A-Sketch, the toy where users rotate two knobs to draw black lines on an off-white screen, and then shake to erase the entire drawing.
At first glance, both the Etch-A-Sketch and an e-reader might look similar, but only on the outside. The gray screen of a blank Etch-A-Sketch comes from the gray-colored aluminum powder coating the inside of the screen. A stylus, controlled by the two white knobs, chisels a thin line through this inner layer of powder. This chisel-action draws lines onto the screen, like children writing in the snow on a car window. Shaking the toy redistributes the powder, restoring a blank slate that can be drawn on again and again. (The sound of maracas that accompanies the shaking, in case you’re wondering, comes from tiny, plastic balls that pound and roll against the aluminum powder to smooth it out.)
There are little balls in e-readers, too, and there they play a much bigger role. Rather than traveling behind the screen, a grid of these hollow balls, called microcapsules, are the pixels of the screen itself. Each one can turn between black and white at a signal (a signal means an input from the reader, such as turning a page), flashing like the bulbs on digital signs. Seen from above, all of the microcapsules—up to about 1.5 million in high-resolution e-readers—line up into picture-perfect blocks of text and whitespace. With that many pixels, each displaying either black or white, the e-reader can display an enormous number of combinations, creating the pages of practically any book, in many assortments of text size and font, with black-and-white illustrations included.
The chameleon-like nature of these microcapsules comes from much smaller spheres within. Hundreds of these tiny black and white spheres swim inside each microcapsule in a sea of liquid plastic. Each sphere is either negatively or positively charged. Running a current over the bigger microcapsules propels the charged spheres within to swim either upwards or downwards. When the bottom of the ball is negatively charged, all of the negatively charged black spheres are repelled. They zip to the top and paint the top of the microcapsule black, while the positively charged white spheres rush to the bottom. When the bottom of the microcapsule is positively charged, white spheres dash up and black ones speed down. (Microcapsules displaying white like this appear to readers as blank space on the e-reader’s screen, like the interior of this letter D.)
Once the spheres, whatever color they are, relax into their current-induced position, they stay there with no further input until a reader turns the page or the screensaver turns on. Each page configuration is in an energy-efficient stasis, which is why an e-reader can last for weeks on a single charge. Like a lounging vacationer, loath to get up from her comfy beach chair unless she really has to, the e-reader stays in this stable, low-energy configuration until it’s told to change the screen.
While an e-reader’s disappearing ink might not be as mystical as the one in Tom Riddle’s diary, it does have real-world advantages. It is convenient, carrying a whole library of books for both ardent and casual readers, and doesn’t use up too much energy. You don’t even need magic to power the e-reader. But perhaps most importantly, there is no fear of inadvertently summoning Voldemort or a deadly basilisk in the process.