Machines that handle information as measurements of physical quantities are called analogue machines, because the measurement is analogous to, or like, the information. A common example of analogue machine is the slide rule. With this we calculate by noting the positions of ruled lines on strips that slide by each other. These strips are made of fine wood, or of plastic, or of steel, in such fashion that the ruled lines will hold true positions and not warp. If we space the rulings so that 1, 2, 3, 4, 5, 6 ··· are equally spaced, then the slide rule is useful for addition ([Fig. 3]). But if we space the rulings so that powers (for example, powers of two—1, 2, 4, 8, 16, 32 ···) ([Fig. 4]) are equally spaced, we can do multiplication. The spacings are then according to the logarithms of numbers ([see Supplement 2]). Multiplication is more troublesome than addition, and so more slide rules are made for multiplication than for addition.
Fig. 1. Measurement by doorpost.
Fig. 2. Measurement by string.
Fig. 3. Slide Rule for adding.
Fig. 4. Slide Rule for multiplying.
During World War II, the aiming and firing of guns against hostile planes was done by machine. After sighting a plane, these machines automatically calculated how to direct fire against it. They were much better and faster than any man. These fire-control instruments were analogue machines with steel and electrical parts built to fine tolerances. With care we can get accuracy of 1 part in 10,000 with analogue machines, but greater accuracy is very hard to get.