Five transmitters are routine tape transmitters. Each of these takes in the sequence of computing steps. The routine tapes also contain information for referring to table tapes and instructions for printing and punching tape. The machine can choose according to instructions between the five routine tapes and can choose between many different sections on each tape. Therefore, we can use a large number of different routines in a calculation, and this capacity makes the machine versatile and powerful. Six transmitters are table tape transmitters. They read tables of numbers when directed to. A table tape can be as long as 100 feet and will hold numbers at the rate of 1 inch per number, so that about 1200 numbers of seven decimal digits can be stored on a table tape. When we look up a number in a table, such as the following, 2½ 3 3½ ··· +———————————————————————————— 1 |1.02500 1.03000 1.03500 2 |1.05063 1.06090 1.07123 3 |1.07689 1.09273 ··· 4 |1.10381 ··· ··· 5 |1.13141 ··· 6 |1.15969 7 | ··· 8 | ··· 9 | ··· 10 | ···| ··· we look along the top and down the side until we find the column and row of the number we are looking for. These are called the _arguments_ of the _tabular value_ that we are looking for (see Supplement 2). Now when we put this table on a tape to go into the Bell Laboratories machine, we write it all on one line, one figure after another, and we punch it as follows: 2-½ 1-5 1.02500 1.05063 1.07689 1.10381 1.13141 6-10 1.15969 ··· 11-15 ··· ··· ··· 3 1-5 1.03000 1.06090 ··· ··· 3½ 1-5 1.03500 ··· You will notice that the column labels 2½, 3, 3½ have been put on the tape, each in front of the group of numbers they apply to. The row labels 1 to 5, 6 to 10, ··· have also been put on the tape, each in front of the group of numbers they apply to. The appropriate column and row numbers, or arguments, must be put often on every table tape, so that it is easy for the machine to tell what part of the table tape it is reading. In the Bell Laboratories machine, we do not need to put equal _blocks_ of arguments like 1-5, 6-10 ··· on the table tape. Instead we can put individual arguments like 1, 2, 3, 4 ···, or, if we wish, we can use blocks of different sizes, like 1-3, 4-15, 16-30···. For some tables, such as income tax tables, it is very useful to have varying-sized blocks of arguments. The machine, when hunting for a certain value in the table, makes a comparison at each block of arguments. The machine needs about 6 seconds to search a foot of tape. If we want to set up a table economically, therefore, we need to consider the average length of time needed for searching. [Illustration: FIG. 1. Scheme of a recorder.] HOW INFORMATION COMES OUT OF THE MACHINE