Question

Why the electrical discharge through the gases could be observed only at very low pressure and very high voltage

Answer 1

Due to the electric field, the gas gets ionised, releasing electrons which move towards the anode, thus constituting the electric current. A low pressure is required as at high pressure the movement of the electrons will be retarded due to collisions with the molecules/atoms of the gas

the passage of electric current through a gaseous medium under the action of an electric field, accompanied by changes in the state of the gas.

Electric discharge in gases occurs when electric current flows through a gaseous medium due to ionization of the gas

As the applied voltage is increased, the free electrons carrying the current gain enough energy to cause further ionization