The UNIVAC II computer was an improvement to the UNIVAC I that the UNIVAC division of Sperry Rand first delivered in 1958. The improvements included the expansion of core memory from 2,000 to 10,000 words; UNISERVO II tape drives, which could use either the old UNIVAC I metal tapes or the new PET tapes; and some transistorized circuits (although it was still overwhelmingly a vacuum tube computer). It was fully compatible with existing UNIVAC I programs for both code and data. It weighed about 16,000 pounds (8.0 short tons; 7.3 t).