Why is the e/m value of cathode rays fixed but that of anode rays change with change of elements?

Anode rays are produced when a low pressure and high voltage is given to a gas. The mass of anode ray particles depend on the gas taken in the tube. Hence , charge / mass ratio changes.


Cathode ray particles simply electrons. Their charge and mass of electrons are independent of the gas taken in the discharge tube. So, charge /mass ratio remain constant.