i learn that, the Symbolics lisp machine's Space-cadet Keyboard uses Honeywell Hall Effect key switch, which is the highest quality key switch.
According to: http://deskthority.net/wiki/Honeywell_Hall_Effect:
Introduced in 1968, Micro Switch (a division of Honeywell) adapted their Hall effect switch technology to work in keyboards. These switches are among the highest quality ever produced by any keyboard switch company, with reliability being paramount to their design. For increased reliability, there are both 3 pin and 4 pin designs of the switch. The 4 pin are +5, GND and dual open-collector outputs, allowing wired-or of the X, Y keyboard matrix. These switches are naturally bounce-free!
The patent itself originates from 1969, but the design is aesthetically very different from the produced switches.
The Honeywell Hall Effect switch is most famous on some Symbolics keyboards, as well as the Space-cadet keyboard.
Keyfeel: Smooth linear switch, makes Cherry MX Black switches feel like they have sandpaper on the stems.
thanks to Yuriy Pitometsu