It always have to be round?

Correct.

Never rectangle.

Never.

Why not?

All the basic idea to that very very microscopic of the gas, the gas, that volumn the reason why it is define as the volumn, its to contain only the gas. Not the liquid, not the solid. So the molecules of the gas are always you imagine: O2, CO2. Look at their basic molecular structure in 3D.

Because... 

The gas molecule, they bounce. Its a free zone, they bounce. I guessing it. It is always round bulb looking. Look at Tesla's lab, 2 bulbs. I made into the video similar AGT that one, 2 light in my hands. Its not the round neon light filling it. The night neon striper night club lights.

The color of those light I think its from the gas. Or the original of....whichever I was taught. But Tesla would be having the first Arc light company before he gone to this !893, where his invention would be the AC, that is a current, not the light itself. You making the light tubes and create a light bulb like Edison. Its the inside material that burn, and he finds 99 kinds, one succeed. Edison ....

AC its the current, like DC and AC Current on War. Its a current.

The electric current. I 

I = current. (Capital letter i )