Understanding the IBM Selectric typewriter

The classic typewriter used physical arms that struck the page through an inked ribbon, leaving a fully formed character. This was simple but slow, and could jam if you typed too quickly and pressed two keys in too-rapid succession. 

The advent of the electric typewriter brought with it several innovations, such as the daisy wheel. This rotated a wheel with characters at the end of each spoke; when the right character was in position, a solenoid struck the wheel’s spoke, printing a fully formed character on the page. This was a very reliable method that avoided jams, and the wheel could be replaced with another to support different typefaces, although it could have a noticeable delay as the daisy wheel spun to each position before printing.

It's not really an overstatement that the IBM Selectric typewriter changed the electric typewriter landscape. The Selectric sported an easily replaceable aluminum ball that rotated and tilted very quickly to position and print a character. The Selectric ball was notable for its speed; the ball's movement was often too fast to be seen. 

The engineering behind the Selectric ball is also remarkable for its clever use of an early digital-to-analog converter. A binary signal of ones and zeroes controlled both the tilt and rotation of the ball, using a physical mechanism called the Whiffle Tree. 

This 2010 video by Bill Hammack, aka the Engineer Guy, explains how the Selectric used these signals and a series of levers and wires to position the ball so quickly and reliably. It's a great watch for anyone who remembers typing on a Selectric.