The paper describes the principles of operation and the construction of storage units with a very short access time for reading. One form of the store which has been constructed has a capacity of 200 000 bits of permanent information and another has been built with a capacity of 100 000 bits, the whole contents of which may be changed in under one minute. The technique employed permits the construction of very large stores at low cost. Each digit cell is formed, basically, by two sets of windings which form the primary and secondary of a transformer and the two binary states are determined by the presence or absence of a piece of linear ferrite material coupling the windings. The access time is largely determined by the physical size of the store and the speed of operation of the associated circuits; a time of 100 millimicrosec is typical.