A few observers have reported the discharge of positive electricity produced by ultra-violet light.Dember found that a metal plate exposed to ultra-violet light in vacuum had the power to communicate a positive charge to a receiving cylinder when the proper accelerating field was applied.His experiments have recently been repeated by DuBridge, who reports that Dember’s results can be entirely accounted for on the basis of the photo-electric effect produced on the collecting cylinder by scattered ultra-violet light against which no precautions had been taken.His conclusions are "there is no measurable photo-electric production of either gaseous or metallic positive ions by the range of wave lengths emitted by the quartz mercury lamp.
R. Bar and F. Luchsinger working with small particles of paraffin and selenium in a Millikan electron apparatus found that some of the particles showed the inverse effect, that is, they became charged negatively when illuminated by ultra-violet light.This has been construed by some workers as indicating the discharge of positive electricity.However, as these men were only using the ultra-violet light to charge up their particles in order to measure the charge communicated, and were not interested in the source of the photo-electric effect it is natural to assume that no special precautions were taken against the light striking the metallic plates of the condenser.The photo-electrons liberated from these metal surfaces being caught by the paraffin and selenium particles would make it seem as if the particles had lost a positive charge. S. Taubes, working in the same laboratory as Luchsinger and Bar and presumably with the same apparatus, reported this inverse effect when dealing with particles that had a large positive charge and showed that it is due to the photo-electrons given off by the condenser plates, for when these were coated with paraffin the inverse effect entirely disappeared.
M. Hake, working at Vienna and using a diminutive Millikan electron apparatus designed by Professor Ehrenhaft, has recently reported results on nineteen different materials.He found that metals show both the inverse and the normal effect, depending on the gas in which the metal particles are suspended, and that some insulators, for example glycerine, always show the inverse effect in all gases.Wasser, working in the same laboratory as Hake gives results on mercury drops in N and CO2.He claims that drops of larger radius than 1.9 x 10-5 cm. always show the normal effect, that those between 1.1 and 1.9 x 10-5 cm. show both normal and the inverse effect, while those of smaller radius than 1.1 x 10-5 cm. always show the inverse effect.
The following investigation was undertaken to test the correctness of the foregoing conclusions and to determine whether the changes in charge experienced by minute mercury droplets are of a magnitude corresponding to the electronic charge or to a charge smaller than this, as the Vienna physicists have maintained.
