An Edisonian Moment

We are told that shortly after Edison invented the light bulb, he handed the glass section of a light bulb to one of his engineers, asking him to find the volume of the inside. This was quite a challenge to the engineer, because a light bulb is such an irregular shape. Figuring the volume of the bulb’s irregular shape was quite different from figuring the volume of a glass, or a cylinder.

Several days later, Edison passed the engineer’s desk, and asked for the volume of the bulb, but the engineer didn’t have it. He had been trying to figure the volume mathematically, and had some problems because the shape was so irregular.

Edison took the bulb from the man; filled the bulb with water; poured the water into a beaker, which measured the volume, and handed it to the amazed engineer.

Vincent A. Miller, Working with Words, Words to Work With, 2001, pp. 57-58

Analyze This!” shows the equilibrium point (x_*, y_*) of

\begin{cases}\frac{dx}{dt}=n k_2 y - n k_1 x^n\\ \frac{dy}{dt}=k_1 x^n-k_2 y\\x(0)=x_0, y(0)=y_0\;\end{cases}

can be found by solving equation


where k_1>0, k_2>0, c_0>0.

When n=4, Omega CAS Explorer‘s equation solver yields a list of x‘s (see Fig. 1).

Fig. 1

It appears that identifying x_* from this list of formidable looking expressions is tedious at best and close to impossible at worst:

Fig. 2

However, by Descartes’ rule of signs ,

\forall k_1>0, k_2>0, c_0>0, -4k_1x^4-k_2x+c_0=0 has exactly one positive root.

It means that x_* can be seen quickly from evaluating the x‘s numerically with arbitrary chosen positive values of k_1, k_2and c_0. For instance, k_1=1, k_2=1, c_0=1 (see Fig. 3).

Fig. 3

Only the second value is positive:

Therefore, x_* is the second on the list in Fig. 1:

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