CONTACT ELECTRIFICATION OF ALKALI-HALIDES

A recently cleaned metal transfers strong positive charge to the surface of an alkali halide crystal. But a metal which has made several (3-30) previous contacts to the alkali halide transfers a much smaller and usually negative charge. If measurements are made in a gas rather than in vacuum the effects of prior contacts may be less pronounced but they are still very noticeable. Experiments on the contact electrification of alkali halides by metals must be interpreted with the possibility of these effects in mind.