Direct measurement of the long-range van der Waals forces

There exists a serious discrepancy in published values for the directly determined van der Waals force between macroscopic bodies, the results of Derjaguin & Abricossova (1951-56) being smaller by a factor of approximately 500 than those of Overbeek & Sparnaay (1951-54). This could have important implications in the quantitative theory of colloids. Some independent determinations have therefore been carried out to resolve this anomaly. Determinations of the force of attraction between parallel, optically flat glass plates in vacuo have been carried out at distances of separation ranging from 0$\cdot $7 to 1$\cdot $2 $\mu $. The method was similar in principle to that of Overbeek & Sparnaay but of improved sensitivity. The force, which ranged from 10$^{-2}$ to 10$^{-3}$ dyn cm$^{-2}$, was determined by measuring the deflexion of a spring, using an electrical capacity method; the distance between the plates was obtained from interference colours produced from the reflexion of white light. Particular attention was paid to elimination of any effect of electrostatic charges on the plates. The results definitely confirm the work of Derjaguin & Abricossova. The agreement is particularly satisfactory in that a different experimental method was employed, with bodies of different shape and with different methods of discharging the plates. Further, the results prove that the retarded dispersion force is operative at the above distances.