The Metrovick 950 was a transistorized computer, built from 1956 onwards by British company Metropolitan-Vickers, to the extent of six or seven machines, which were "used commercially within the company" or "mainly for internal use". The 950 appears to have been Metrovick's first and last commercial computer offering. Transistor Computer Prototype In November 1953 Richard Grimsdale and Douglas Webb of Manchester University first demonstrated their prototype transistorized computer using 92 point-contact transistors and 550 diodes in order to test the suitability of transistors in improving the reliability of the Manchester Mark 1 computer. This machine was similar to the Mark I, except that it did not include Williams tubes and used only the magnetic drum for main memory. The machine was based on a 48-bit word, although four bits were used for timing and thus not available for program use. This machine used thermionic valves to generate a clock frequency of 125 kHz. The only storage used was a drum (reused from the Manchester Mark 1). This meant that the average random access time to a word in store was half a drum revolution, i.e., with 64 words on a track, 32 times the random access time for a word if it could be stored in a true RAM. In comparison, the Mark 1 had also included a number of Williams tubes to provide computer registers and dramatically speed access to a small amount of data; thus the Transistor Computer was slower than the Mark 1. Both versions had a pseudo 2-address (or 1+1) instruction format, where the address of the next instruction to be obeyed was contained within each instruction, to facilitate "optimum programming". The drum was even used to store the Accumulator and the Current Instruction. Improved version By 1955 the design was re-built with a Mark I-style B-line accumulator/index register and hardware multiplier. The added circuitry brought the machine to a total of 250 transistors. It used only 150 watts of power, an astonishingly low figure in an era when machines typically used tens of kilowatts to warm their valves. The new model could add two 44-bit numbers in 1.5 drum revolutions, which, at a drum spin rate of 3000 RPM, amounted to roughly 30 milliseconds per addition. Although faster than the Mark I internally, the lack of Williams tubes made it run much more slowly; the Mark I could add two 40-bit numbers in 1.8 milliseconds.