The numbers that the Bell machine contains range from 0.1000000 to 0.9999999 times a _power_ of 10 varying from 10,000,000,000,000,000,000 to 0.000,000,000,000,000,000,1, or, in other words, from 10¹⁹ to 10⁻¹⁹. The machine also contains zero and _infinity_: zero arises when the number is smaller than 10⁻¹⁹, and infinity arises when the number is equal to or greater than 9,999,999,000,000,000,000. (See Supplement 2.) The system used in the machine to represent numbers on relays is called _biquinary_—the _bi_-, because it is partly twofold like the hands, and the -_quinary_ because it is partly fivefold like the fingers. This system is used in the abacus (see Chapter 2 and Supplement 2). In the machine, for each decimal digit, 7 relays are used. These relays are called the 00 and 5 relays, and the 0, 1, 2, 3, and 4 relays. If, as before, 0 indicates a relay that is not energized and 1 indicates a relay that is energized, then each decimal digit is represented by the positioning of the 7 relays as follows: DECIMAL DIGIT RELAYS 00 5 0 1 2 3 4 0 1 0 1 0 0 0 0 1 1 0 0 1 0 0 0 2 1 0 0 0 1 0 0 3 1 0 0 0 0 1 0 4 1 0 0 0 0 0 1 5 0 1 1 0 0 0 0 6 0 1 0 1 0 0 0 7 0 1 0 0 1 0 0 8 0 1 0 0 0 1 0 9 0 1 0 0 0 0 1 Then, for any decimal digit, one and only one of the 00 and 5 relays is energized, and one and only one of the 0, 1, 2, 3, and 4 relays is energized. If more or less than exactly one relay in each set is energized, then the machine knows that it has made a mistake, and it stops dead in its tracks. Thus any accidental failure of a relay is at once caught, and the chance of two compensating failures occurring at the same time is extremely small. HOW INFORMATION GOES INTO THE MACHINE In order to put a problem into this machine—just as with the other machines—first a mathematician who knows how the problem is to be solved, and who knows how to organize it for the machine, lays out the scheme of calculation. Then, a girl goes to one of the hand perforators. Sitting at the keyboard, she presses keys and punches out feet or yards of paper tape expressing the instructions and numbers for the calculation. Each character punched—digit, letter, or sign—has one or more of a maximum of 6 holes across the tape. Another girl, using the other hand perforator, also punches out the instructions and numbers for the calculation. If she wishes to erase a wrong character, she can press an _erase key_ that punches all 6 holes, and then the machine will pass by this row as if it were not there. Three kinds of tapes are produced for the machine: _Problem tapes_, which contain information belonging to the particular problem.