Question: Suppose that to create an electron filter you modify the setup shown in Fig. by drilling another small hole in the left-hand plate opposite the hole in the right-hand plate. (The holes are too small to have much effect on the electric field between the plates.) Suppose that you want to set up this apparatus so that when electrons are fired towards the holes, any electrons with speeds less than 107 m s-1 will be slowed down by the electric field between the plates and will not emerge from the other side, while electrons with higher speeds will emerge through the hole. Assuming your voltage source can be turned up as high as 500V, how would you adjust your apparatus to achieve your goal? Be sure to specify the distance between the plates and the polarity of the voltage.

Fig. An arrangement of charged plates for accelerating positively charged ions.