Now we need a way of putting any function we wish into a differential analyzer. To do this, we use a mechanism called a _function table_. We draw a careful graph of the function according to the scale we wish to use, and we set the graph on the outside of a large drum (Fig. 22). For example, we can put the resistance coefficient graph on the drum; the speed (or _independent variable_) goes around the drum, and the resistance coefficient (or _dependent variable_) goes along the drum. The machine slowly turns the drum, as may be called for by the problem. A girl sits at the function table and watches, turning a handwheel that keeps the sighting circle of a pointer right over the graph. The turning of the handwheel puts the graphed function into the machine. Instead of employing a person, we can make one side of the graph black, leaving the other side white, and put in a _phototube_ (an electronic tube sensitive to amount of light) that will steer from pure black or pure white to half and half (see Fig. 23). We do not need many function tables to put in information, because we can often use integrators in neat combinations to avoid them. We shall say more about this possibility later. We can also use a function table to put out information and to draw a graph. To do so, we disconnect the handwheel; we connect the shaft of the handwheel to the shaft that records the function we are interested in; we take out the pointer and put in a pen; and we put a blank sheet of graph paper around the drum. [Illustration: FIG. 23. Phototube following graph.] We have now described the main parts of the second MIT differential analyzer. They are the parts that handle numbers. We can now tell the capacity of the differential analyzer by telling the number of main parts that it holds: 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.