action due to the movement of the ions in an electric field. The recoil, due to the expulsion of α particles from one side of the vane, is far too small to account for the movement observed.
This effect can, I think, be simply accounted for by taking into consideration the difference in conductivity of the gas on the two sides of the radium coated vane. If a small vane, coated uniformly with radium on both sides, and mounted on an insulating support, be brought near a charged body kept at a constant potential, it acts like a water dropper and rapidly acquires very nearly the average potential which existed at that point before the vane was brought up. The mechanical force acting on the vane will, in consequence, be small. If, however, the vane is only coated with radium on the side near the charged body, the ionization and consequently the conductivity of the gas is much greater between the vane and the charged body than on the opposite side. Suppose, for simplicity, the body is charged to a positive potential. On account of the greater conductivity of the gas on the side facing the charged body, it will rapidly acquire a positive charge, and the potential of the vane will reach a higher value than existed at that place before the vane was introduced. This will result in a repulsion of the vane. This also accounts for the attraction observed in the experiment with the Coulomb's balance already referred to. Suppose that one sphere is positively charged and the other earthed, and the two vanes metallically connected together. The vane next to the charged body will become charged positively, but this charge will be dissipated rapidly on account of the ionization of the gas close to the opposite vane, and, in most conditions, this loss of charge will be so rapid that the potential of the vane is unable to reach the value which would exist at that place in the field, if the vane were removed. There will, in consequence, be an attracting force acting on the vane towards the sphere.
The repulsion observed by Joly is thus only an indirect result of the ionization in the gas produced by the radium, and should be shown under conditions where similar unequal distribution of ionization is produced by any other sources.
Since radium gives out heat at a fairly rapid rate, a radiometer in which the vanes were coated on one side with radium instead of lampblack, should rotate at low pressure of the gas, even if no
