In reading over the table, we find a number of words that need explaining. Some of the explanation we can give in the summary of the units of Eniac:
SUMMARY OF UNITS OF ENIAC
| Quantity | Device | Significance |
|---|---|---|
| 20 | Accumulators | Store, add, and subtract numbers |
| 1 | Multiplier | Multiplies |
| 1 | Divider-Square-Rooter | Divides, and obtains twice the square root |
| of a number ([see Supplement 2]) | ||
| 3 | Function Tables | Part of the memory, for referring to |
| tables of numbers | ||
| 1 | Constant Transmitter | Stores numbers from the card reader and |
| from hand-set switches | ||
| 1 | Printer | Punches machine results into cards |
| 1 | Cycling Unit | Controls the timing of the various parts |
| of the machine | ||
| 1 | Initiating Unit | Has controls for starting a calculation, |
| for clearing, etc. | ||
| 1 | Master Programmer | Holds the chief controls for coordinating |
| the various parts of the machine |
An accumulator is a storage counter. It can hold a number; it can clear a number; it can transmit a number either positively or negatively; and it can receive a number by adding the number in and thus holding the sum of what it held before and the number received. Eniac when first built had only 20 accumulators, and so it could remember only 20 numbers at one time (except for constant numbers set in switches). This small memory was the most serious drawback of Eniac; panel 26 was designed, therefore, to provide a great additional memory capacity.
The divider-square-rooter, as its name tells, is a mechanism that can divide and that can find twice the square root of a number. Eniac is one of the several giant brains that have had square root capacity built into them, particularly since square root is needed for solving trajectories.
Many panels of Eniac have double duty and some have triple duty. For example, panel 24 is an accumulator, but it also (1) stores the right-hand partial products ([see Supplement 2]) of the multiplier and (2) was a register, when Eniac was at Moore School, from which information to be punched in the printer could be obtained. Clearly, if we have a multiplication to do, we cannot also use this accumulator for storing a number that is to remain unchanged during the multiplication.
Parts
The total number of parts in Eniac is near half a million, even if we count electronic tubes as single parts. There are over 18,800 electronic tubes in the machine. It is interesting to note that only 10 kinds of electronic tubes are used in the calculating circuits and only about 60 kinds of resistors and 30 kinds of capacitors. A resistor is a device that opposes the steady flow of electric current through it to a certain extent (called resistance and measured in ohms). A capacitor is a device that can store electrical energy up to a certain extent (called capacitance and measured in farads). All these tubes and parts are standard parts in radios. All types are identified by the color labels established in standard radio manufacturing. It is the combinations of these parts, like the combinations of pieces in a chess game, that give rise to the marvelous powers of Eniac.
The combinations of parts at the first level are called plug-in units. A plug-in unit is a standard box or tray or chassis made of sheet steel containing a standard assembly of tubes, wires, and other parts. It can be pushed in or pulled out of a standard socket with many connections. An example of a plug-in unit is a decade, or, more exactly, an accumulator decade. This is just a counter wheel or decimal position expressed in Eniac language: it can express successively all the digits from 0 to 9 and then pass from 9 to 0, giving rise to a carry impulse. It is striking that a mechanical counter to hold 10 digits can be made up of 10 little wheels, ¼ inch wide and an inch high. But an accumulator in Eniac, to hold 10 digits, is a set of 10 decades each 2 inches wide and 2½ feet high.
There are only about 20 kinds of plug-in units altogether. Each plug-in unit is interchangeable with any other of the same kind. So, if a decade, for example, shows trouble, you can pull it out of its socket and plug in a spare decade instead.