Question

when cathode discharge tube experiment is perforned under high voltage, why doesn't the glass tube melt ?

Answer 1

The cathode ray tubes used today differ from the Hittorf-Crooker’s tube used for experimentation by Thompson, Rontgen and other scientists.
The newer tubes use a metal filament (made of tungsten or tungsten alloy) which is heated to generate electrons and this process generates heat. The new tubes give a better control over the number of electrons produced. The cathode ray tubes used for experimentation by scientists were cold electrodes. These were partially evacuated glass tubes with two electrodes and a gas. The electrodes were maintained at a high potential difference and electrons and gas provided the electrons. A heated metal filament was not the source of electrons and therefore not much of heat was generated in these cathode ray tubes.