Page:Radio-activity.djvu/71

 The condensation vessel was similar to that employed by C. T. R. Wilson. Two parallel horizontal plates were fitted in the vessel and the radiation from an X ray tube or radio-active substance ionized the gas between them. A difference of potential V, small compared with that required to saturate the gas, was applied between the parallel plates distant l cms. from each other. The small current i through the gas is given (section 28) by

i = NuVe/l,

where N = number of ions present in the gas,

e = charge on each ion, u = sum of the velocities of the positive and negative ions.

Since the value of N is the same as the number of drops, and the velocity u is known, the value of e can be determined.

In his last determination J. J. Thomson found that

e = 3·4 × 10^{-10} electrostatic units.

A very concordant value, namely, 3·1 × 10^{-10}, has been obtained by H. A. Wilson, by using a modified method of counting the drops. A check on the size of the drops, determined by their rate of fall, was made by observing the rate at which the drops moved in a strong electric field, arranged so as to act with or against gravity.

J. J. Thomson found that the charge on the ions produced in hydrogen and oxygen is the same. This shows that the nature of the ionization in gases is distinct from that occurring in the electrolysis of solutions where the oxygen ion always carries twice the charge of the hydrogen ion.

37. Diffusion of the ions. Early experiments with ionized gases showed that the conductivity was removed from the gas by passage through a finely divided substance like cotton-wool, or by bubbling through water. This loss of conductivity is due to the fact that the ions in passing through narrow spaces diffuse to the sides of the boundary, to which they either adhere or give up their charge.

A direct determination of the coefficient of diffusion of the ions