More on electrical contacts from our good friend Wikipedia.
https://en.m.wikipedia.org/wiki/Electrical_contacts
Excerpt:
Electrical contact theory Edit
Ragnar Holm contributed greatly to electrical contact theory and application.[10]
Macroscopically smooth and clean surfaces are microscopically rough and, in air, contaminated with oxides, adsorbed water vapor, and atmospheric contaminants. When two metal electrical contacts touch, the actual metal-to-metal contact area is small compared to the total contact-to-contact area physically touching. In electrical contact theory, the relatively small area where electrical current flows between two contacts is called the a-spot where "a" stands for asperity. If the small a-spot is treated as a circular area and the resistivity of the metal is homogeneous, then the current and voltage in the metal conductor has spherical symmetry and a simple calculation can relate the size of the a-spot to the resistance of the electrical contact interface. If there is metal-to-metal contact between electrical contacts, then the electrical contact resistance (as opposed to the bulk resistance of the contact metal) is mostly due to constriction of the current through a very small area, the a-spot. Contact force or pressure increases the size of the a-spot which decreases the constriction resistance and the electrical contact resistance.[11] When the size of contacting asperities becomes larger than the mean free path of electrons, Holm-type contacts become the dominant transport mechanism, resulting in a relative low contact resistance.[12]
https://en.m.wikipedia.org/wiki/Electrical_contacts
Excerpt:
Electrical contact theory Edit
Ragnar Holm contributed greatly to electrical contact theory and application.[10]
Macroscopically smooth and clean surfaces are microscopically rough and, in air, contaminated with oxides, adsorbed water vapor, and atmospheric contaminants. When two metal electrical contacts touch, the actual metal-to-metal contact area is small compared to the total contact-to-contact area physically touching. In electrical contact theory, the relatively small area where electrical current flows between two contacts is called the a-spot where "a" stands for asperity. If the small a-spot is treated as a circular area and the resistivity of the metal is homogeneous, then the current and voltage in the metal conductor has spherical symmetry and a simple calculation can relate the size of the a-spot to the resistance of the electrical contact interface. If there is metal-to-metal contact between electrical contacts, then the electrical contact resistance (as opposed to the bulk resistance of the contact metal) is mostly due to constriction of the current through a very small area, the a-spot. Contact force or pressure increases the size of the a-spot which decreases the constriction resistance and the electrical contact resistance.[11] When the size of contacting asperities becomes larger than the mean free path of electrons, Holm-type contacts become the dominant transport mechanism, resulting in a relative low contact resistance.[12]