Thanks!

Just to clarify about the borosilicate glass, it is used because it is stronger and more resistant to thermal shock (and thermally shocked those tubes will be). Tempered glass is mostly used where safety is a main concern, because it breaks into small pieces, that are not as sharp as other types of glass. That makes it [tempered] ideal for car windows, and glass that protects someone; but not for applications where rapid heating and cooling may be an issue. More information here: itstillruns.com/borosilicate-vs-tempered-glass-5786607.html

Good presentation but it would be improved if you used "Dewar" flask instead of "Thermos" since Sir James Dewar made the first practical vacuum flask. The heat retention is of course achieved by eliminating the heat transferred by convection currents, and further improved (in a Dewar flask) by the silvering of interior surfaces which minimize radiation transfer. Not having seen an evacuated hot water tube, I do not know if they have interior silvering - probably not as they need to transfer energy to the core from the exterior.

I want this kind of video 4k :))

Bora-silica glass. Does that require sand to make?

Where is the sound?

try this one for a concise explanation: hydrosolar.ca/blogs/news/how-do-vacuum-tubes-collector-work