The kinetic energy of an electron in a photoelectric tube increases with increase in the applied voltage across the plates of the tube, thus the velocity of the electrons also increases. Accordingly the time taken by the electron to reach the opposite plate should be less and so the current in the external circuit should increase as $i=\frac{dp}{dt}$ and so current is inversely proportional to time.But this is not the case as the value of saturation current remains the same.Why is the value not depending on the time taken by the electron to reach the opposite plate?
Answer
The current in any circuit is given by i= dq/dt. q being the charge of the carriers not dp/dt. Increasing the voltage will only increase the kinetic energy of the electrons coming out but not the current because the number of electrons available is the same(unless you are varying the intensity of the light which is creating the effect).
But if their is air, it should resist the electron flow a little. Hence you might see a voltage dependence. But that possibly wouldn't be ohmic.
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