In 1932 Tauschek obtained a US patent for his magnetic drum device
n its most basic form, magnetic drum memory is simply a metal drum or cylinder, coated with a ferromagnetic recording material. Stationary write heads emit an electrical pulse, changing the magnetic orientation of a particle at a given position on the drum. The read heads, which are also stationary, recognize a particle’s orientation as either a binary 1 or 0. Tauschek’s prototype could store 500000 bits across the drum’s total surface for a capacity of about 62.5 KB.
The principal difference between a drum as described and a modern disk is that on a drum the heads do not have to move to the track to access, as the controller simply waits for the data to appear under the relevant head as the drum turns. In a disk drive the head takes a certain time, the seek time, to move into place, while the performance of a drum with fixed heads is determined almost entirely by the rotational speed.
As late as 1980, the PDP-11/45 machines that used drums for swapping were still in use at many of the original UNIX sites. In modern-day BSD Unix and its descendants, /dev/drum is the name of the default swap device, deriving from the use of drum secondary-storage devices as backing store for pages in virtual memory.