Page:Radio-activity.djvu/73

 The moist gases were saturated with water vapour at a temperature of 15° C.

It is seen that the negative ion in all cases diffuses faster than the positive. Theory shows that the coefficients of diffusion should be directly proportional to the velocities of the ions, so that this result is in agreement with the observations on the greater velocity of the negative ion.

This difference in the rate of diffusion of the ions at once explains an interesting experimental result. If ionized gases are blown through a metal tube, the tube gains a negative charge while the gas itself retains a positive charge. The number of positive and negative ions present in the gas is originally the same, but, in consequence of the more rapid diffusion of the negative ions, more of the negative ions than of the positive give up their charges to the tube. The tube consequently gains a negative and the gas a positive charge.

38. A very important result can be deduced at once when the velocities and coefficients of diffusion of the ions are known. Townsend (loc. cit.) has shown that the equation of their motion is expressed by the formula

(1/K)pu = -dp/dx + nXe,

where e is the charge on an ion,

n = number of ions per c.c., p = their partial pressure,

and u is the velocity due to the electric force X in the direction of the axis of x. When a steady state is reached,

dp/dx = 0 and u = n X e K/p.

Let N be the number of molecules in a cubic centimetre of gas at the pressure P and at the temperature 15° C., for which the values of u and K have been determined. Then N/P may be substituted for n/p, and, since P at atmospheric pressure is 10^6,