In the mid-1970s, the Soviet Union developed and deployed a hardware keylogger targeting typewriters. Termed the "selectric bug", it measured the movements of the print head of IBM Selectric typewriters via subtle influences on the regional magnetic field caused by the rotation and movements of the print head. An early keylogger was written by Perry Kivolowitz and posted to the Usenet newsgroup net.unix-wizards, net.sources on November 17, 1983. The posting seems to be a motivating factor in restricting access to /dev/kmem on Unix systems. The user-mode program operated by locating and dumping character lists (clients) as they were assembled in the Unix kernel.