As the preceding Figure 1 shows, the experiment at the exhibit “The Compressed Circle” consists of bending up a circular spring by means of two (initially vertical and parallel) rotatable levers in such a way that its shape in the final position (see Figure 2 below) represents a distance from point to point .
is the length of the circumference of the circle that the spring originally formed. Moreover, the area of the right triangle with vertices , and (see Figure 1) is equal to half the area of the circle, i.e.
Thus, the area of a circle with radius can be represented by the sum of the areas of two congruent triangles.
And now … the mathematics of it:
If the two levers are turned by the angle against the vertical axis, the contact point between the right lever arm and the bent-up circle results on the right side. The latter now represents itself as a circular arc with the radius and the opening angle (in radians!). Thus
According to Figure 3 above, for the straight lines and ,
Their intersection is then obtained as the solution of the equation
According to equation (1), this results in
Equations (1) and (2) then yield
which, after multiplication out and truncation, leads to
leads. This is now called
Consequently, without restriction, it can be assumed that , so that for a given opening angle (in radians) of the right lever (see Figure 3), the opening angle of the corresponding circular arc (with radius ) is obtained as the solution of the following equation:
Finally, we give — determined numerically as approximations — for () the corresponding angles and the radii (using the above equation).
Figure 4 below summarizes this in a diagram.