The air between two parallel plates separated by a distance `d = 20 mm` is inoized by X-ray radisation. Each plate has an area `S = 500 cm^(2)`. Find the concentration of positive ions if at a voltage `V = 100 V` a current `I = 30 muA` flows between the paltes, which is well below the saturation current. The air ion mobilities are `u_(0)^(+) = 1.37 cm^(2)//(V .s)` and `u_(0) = 1.91 xm^(2)//(V . s)`