We don’t know why [TubeTime] decided to show off this oddball keyboard switch as a series of Twitter posts, but we were glad to see them somewhere. At first, the switch looks pretty conventional. But as the pictures reveal the insides, you’ll notice something unusual: a ferrite toroid! These switches operate as a transformer and are known as magnetic valve switches.
The switches have two sets of two pins — one set for the primary and one for the secondary of the transformer wound around the ferrite core. That transformer remains stationary, but a pair of permanent magnets move. When the key is up, the magnets are close to the core and cause the transformer to saturate, so there is little or no output at the secondary. When you depress the key, the magnet moves away from the core, allowing the signal to pass through the transformer. What that means is there is no mechanical contact, which is good for switch life. It is also important in environments where a small spark could cause an explosion. You can watch a video about a keyboard that used those switches, below.
Sometimes known as Cortron switches, they appeared as a patent in 1974. While they did find use in quite a few products — especially those where a mechanical switch was undesirable, hall effect switches became more economical to produce and drove the magnetic valve switches out of the market by 2005.
Surprisingly, this is one of those rare topics where an Internet search only turns up a handful of pages. There’s a brief mention of their history on a keyboard site (that also has an interesting discussion about interfacing). We also found some information about where they turned up on this page. Those, along with a few stray discussion posts and the patents, seemed to be about it.