The contact potential difference or Volta potential

Abstract

Sometime in 1801, Volta, who, together with Galvani, was responsible for the development of the galvanic cell, Voltaic pile, or, in short, the electrolytic cell, believed that there was an intrinsic potential difference between dry metals in no way related to the electrolytic phenomena occurring between metals in solutions. He even went so far as to arrange the dry metals in a potential series based on some simple electrostatic experiments. Volta’s belief also supported by Helmholtz ³⁴ went largely unrecognized for many years through confusion of the phenomenon with the electrolytic actions also earlier discovered by Volta as well as with the Peltier effect potentials. Controversy was rife for many years until finally in 1898, Lord Kelvin ³⁵ presented to the Royal Institution and published in the Philosophical Magazine his studies carried forward from 1859 to 1861 onward that definitely established the existence of an intrinsic contact difference of potential between different metals. The potentials bore no relation to electrochemical effects and were independent of the medium in which the metals were immersed as long as the metals were not altered by the medium. More clarification relating to the contact or Volta difference of potential came from the basic studies of R. A. Millikan ³⁶ in 1915 on the Einstein photoelectric law in which he identified this quantity with the difference between the work functions of the two metals in question. Using light on various metal surfaces shaved in vacuum, he established the energy required to remove electrons from different metals and showed that this energy difference represented the differences in work function, for free electrons in the metal, without any additional work being required to remove the electrons from atoms in the metal. Actually, at an earlier date, the electron theory of metals set up by H. A. Lorentz after J. J. Thomson’s identification of the electron led those active in the field to accept a characteristic work function needed to remove electrons from metals. Thus as early as 1901, H. A. Wilson ³⁷ and later, Wilson and O. W. Richardson, set up thermodynamic analyses of the thermionic current of electrons liberated by hot bodies following J. J. Thomson’s proof that these particles were electrons in 1899. Experimentally, O. W. Richardson ³⁸ in 1901 began his measurements of thermionic emission as a function of temperature to test the theory of Wilson. In consequence, by 1916, there was enough data to indicate that in principle the contact potential differences between clean metals were closely related to the thermionically and photoelectrically measured differences in work functions.