The Contact Electricity of Solid Dielectrics

The electric charges produced by wringing optically flat surfaces together were measured in order to determine whether or not there is a possibility of formulating a single contact theory which will include both the metals and the dielectrics. Experiments with flint glass and steel proved that the frictional charge is independent of the amount of friction, provided only that intimate contact be established, and is proportional to the area of contact. The voltaic nature of the frictional charge. The charge was in no wise affected by the ionization of the residual air molecules between the surfaces by means of an intense beam of x-rays, and was also found to be independent of the duration of contact for periods varying up to 17 hours. The failure of the double-layer to recombine under these conditions proves that it was sustained by a voltaic field. The dependence of the effect on the dielectric constant. The charge per cm² Q₁₂ of material 1 in contact with material 2, was found for 8 different pairs of the materials, quartz, fluorite, crown glass, flint glass and steel, to satisfy, within 14 per cent, the equation Q₁₂=C(K₁-K₂), where K₁ and K₂ are the dielectric constants and C is a positive constant, whose mean value is 4.43 e.s.u., provided the value $K=3.1$ be assigned to steel. This equation is consistent with the results of Coehn's measurements of electric osmosis.The electric effect of compressing amorphous dielectrics was determined by pressing two kinds of sheet rubber, of dielectric constants 2.94 and 3.96, against seven hard materials, whose dielectric constants ranged from 2.8 to 7.8. The charge on the compressible dielectric was found to be independent of the nature of the material against which it was pressed, proving that this is not a voltaic effect and that amorphous as well as crystalline substances can be electrified by pressure.The electric effect of collision of a solid insulator and a metal was found, with four pairs of materials, to be consistently opposite in sign to the frictional effect. This result shows that collision must be considered to produce two different effects, one of which is the voltaic charge, while the other is a transfer of electrons from the metal to the dielectric, due in all probability to the inertia of the mobile electrons.Dielectric constant of steel, as suggested by these results, is not infinity but 3.1.