<P> A multiplication of a 10 - digit number by a d - digit number (for d up to 10) took d + 4 cycles, so a 10 - by 10 - digit multiplication took 14 cycles, or 2800 microseconds--a rate of 357 per second . If one of the numbers had fewer than 10 digits, the operation was faster . </P> <P> Division and square roots took 13 (d + 1) cycles, where d is the number of digits in the result (quotient or square root). So a division or square root took up to 143 cycles, or 28,600 microseconds--a rate of 35 per second . (Wilkes 1956: 20 states that a division with a 10 digit quotient required 6 milliseconds .) If the result had fewer than ten digits, it was obtained faster . </P> <P> ENIAC used common octal - base radio tubes of the day; the decimal accumulators were made of 6SN7 flip - flops, while 6L7s, 6SJ7s, 6SA7s and 6AC7s were used in logic functions . Numerous 6L6s and 6V6s served as line drivers to drive pulses through cables between rack assemblies . </P> <P> Several tubes burned out almost every day, leaving ENIAC nonfunctional about half the time . Special high - reliability tubes were not available until 1948 . Most of these failures, however, occurred during the warm - up and cool - down periods, when the tube heaters and cathodes were under the most thermal stress . Engineers reduced ENIAC's tube failures to the more acceptable rate of one tube every two days . According to a 1989 interview with Eckert, "We had a tube fail about every two days and we could locate the problem within 15 minutes ." In 1954, the longest continuous period of operation without a failure was 116 hours--close to five days . </P>

Name the machine that was called the first electro mechanical computer