The SSEM's 32-bit word length was increased to 40 bits. Each word could hold either one 40-bit number or two 20-bit program instructions. The main store consisted of two Williams tubes each holding an array of 32 x 40-bit words – known as a page – backed up by a magnetic drum capable of storing an additional 32 pages; the capacity was increased to 128 pages in the Final Specification version. The 12-inch (300 mm) diameter drum, initially known as a magnetic wheel, contained a series of parallel magnetic tracks around its surface, each with its own read/write head. Each track held 2,560 bits, corresponding to 2 pages (2 x 32 x 40 bits). One revolution of the drum took 30 milliseconds, during which time both pages could be transferred to the CRT main memory, although the actual data transfer time depended on the latency, the time it took for a page to arrive under the read/write head. Writing pages to the drum took about twice as long as reading. The drum's rotational speed was synchronised to the main central processor clock, which allowed for additional drums to be added. Data was recorded onto the drum using a phase modulation technique still known today as Manchester coding.