The C tape puts the initial conditions into the machine. For example, reading the C tape for the problem, the machine finds that 3000 should, at the start of the problem, stand in counter 4. The machine then reads the number at which counter 4 actually stands, say 6728.3. It subtracts the two numbers and remembers the difference, -3728.3. And whenever the machine reads that counter later, finding, say, 9468.4 in it, first the 3728.3 is subtracted, and then the answer 5740.1 is printed.
ANSWERS
Information may come out of the machine in either one of two ways: in printed numbers or in a graph. In fact, the same quantity may come out of the machine in both ways at the same time. To obtain a graph, we change a function table from input to output, put a pen on it, and have it draw the graph.
The machine has 3 electric typewriters. The machine will take numerical information out of the counters at high speed even while they are turning, and it will put the information into relays. Then it will read from the relays into the typewriter keys one by one while they type from left to right across the page.
HOW THE DIFFERENTIAL ANALYZER CALCULATES
Up to this point in this chapter, the author has tried to tell the story of the differential analyzer in plain words. But for reading this section, a little knowledge of calculus is necessary. ([See also Supplement 2].) If you wish, skip this section, and go on to the next one.
We have described how varying quantities, or variables, are operated on in the machine in one way or another: adding, subtracting, multiplying by a constant, referring to a table, and integrating. What do we do if we wish to multiply 2 variables together? A neat trick is to use the formula:
| xy = | ∫ x dy + | ∫ y dx |
To multiply in this way requires 2 integrators and 1 adder. The connections that are made between them are as follows:
| Shaft x | To Integrator 1, Screw |
| Shaft x | To Integrator 2, Disc |
| Shaft y | To Integrator 1, Disc |
| Shaft y | To Integrator 2, Screw |
| Integrator 1, Wheel | To Adder 1, Input 1 |
| Integrator 2, Wheel | To Adder 1, Input 2 |
| Adder 1, Output | To Shaft expressing xy |