In Prob. 12.64, determine the smallest allowable value of the ratio d/l in terms of e, m,...

In Prob. 12.64, determine the smallest allowable value of the ratio d/l in terms of e, m, υ0, and υ if at x =l the minimum permissible distance between the path of the electrons and the positive plate is 0.075d.

Fig. P12.65