Solution

Electrons pass through capacitor if they don't collide with plates. So, supposing a capacitor of infinite length, electron's velocity must be parallel to the plates (or its horizontal range over a plate greater than its enter's height). This is possible if electric and magnetic forces are in equilibrium (equal absolute values and opposed directions).

$$evB = eE$$

$$v = \frac{E}{B}$$

As $E=\frac{V}{h}$,

$$\boxed{v = \frac{V}{Bh}}$$

This device is known as speed's selector.