| Shafts | About 130 |
| One-digit gearboxes | 12 |
| Four-digit gearboxes | 16 |
| Adders | About 16 |
| Integrators | 18 |
| Function tables | 3 |
On a simpler level, we can say that the machine holds these physical parts:
| Miles of wire | About 200 |
| Relays | About 3000 |
| Motors | About 150 |
| Electronic tubes | About 2000 |
INSTRUCTING THE MIT
DIFFERENTIAL ANALYZER NO. 2
Besides the function tables for putting information into the machine, there are three mechanisms that read punched paper tape. The three tapes are called the A tape, the B tape, and the C tape. From these tapes the machine is set up to solve a problem.
Suppose that we have decided how the machine is to solve a problem. Suppose that we know the number of integrators, adders, gearboxes, etc., that must be used, and know how their inputs and outputs are to be connected. To carry out the solution, we now have to put the instructions and numbers into the machine.
The A tape contains instructions for connecting shafts in the machine. Each instruction connects a certain output of one type of mechanism (adder, etc.) to a certain input of another type of mechanism. When the machine reads an instruction on this tape, it connects electrically the transmitting angle-indicator of an output shaft to the receiving angle-indicator of another input shaft.
Now the connecting part of the differential analyzer behaves as if it were very intelligent: it assigns an adder or an integrator or a gearbox, etc., to a new problem only if that mechanism is not busy. For example, if a problem tape calls for adder 3 (in the list belonging to the problem), the machine will assign the first adder that is not busy, perhaps adder 14 (in the machine), to do the work. Each time that adder 3 (in the problem list) is called for in the A tape, the machine remembers that adder 14 was chosen and assigns it over again. This ability, of course, is very useful.
The B tape contains the ratios at which the gearboxes are to be set. For example, suppose that we want gearbox 4 (in the problem list) to change its input by the ratio of 0.2573. The machine, after reading the A tape, has assigned gearbox 11 (in the machine list). Then, when the machine reads the B tape, it sets the ratio in gearbox 11 to 0.2573.
The answer to a differential equation is different for different starting conditions. For example, when we know speed and time and wish to find distance traveled and where we have arrived, we must know the point at which we started. We therefore need to arrange the machine so that we can put in different starting conditions (or different initial conditions, as the mathematician calls them).