Why does charge-to-mass ratio of positive rays depend on the reside gas in the discharge tube ? Why is the charge-to-mass ratio of all cathode rays are same ?
The positive rays are actually positively charged ions (not protons, unless the gas in the discharge tube is hydrogen). When the charge to mass ratio is measured the mass of the moving positive charge will be the mass of the nucleus of whatever gas is in the discharge tube.
Cathode rays are made up of electrons and all electrons have same charge-to-mass ratio. That’s why charge to mass ratio of all cathode rays is same