The UNISERVO tape drive was the primary I/O device on the UNIVAC I computer. Its place in history is assured as it was the first tape drive for a commercially sold computer. The UNISERVO used metal tape: a 1⁄2-inch-wide (13 mm) thin strip of nickel-plated phosphor bronze (called Vicalloy) 1200 feet long. These metal tape reels were very heavy. Data was recorded in eight channels on the tape (six for the data value, one parity channel for error checking, and one timing channel) at a density of 128 bits per inch. The tape could be moved at 100 inches per second, giving a nominal transfer rate of 12,800 characters per second. Data was recorded in fixed size blocks of 60 words of 12 characters each. Making allowance for the empty space between tape blocks, the actual transfer rate was around 7,200 characters per second.