By definition, an electrical contact is a place in an electrical circuit where two or more current carrying conductors get in touch. The contact phenomenon exists not only between two metal pieces but also between a metal body and liquid (electrolyte) or gas in the form of plasma. In electrical engineering predominantly metal contact bodies are used to establish an electrical contact. From terminology viewpoint, when such a system is especially designed, it is named a contact system. There are different contact body configurations. From geometry point of view, as depicted in figure:
there are 3 different types of electrical contacts. For the two hemispheres a) a point contact is established.
A linear contact exists between a cylindrical rod and a plate, while for the two plates (or bars), a plane contact is realized.
In any case, a certain contact pressure (a contact force F_{c}) must be applied to the structure of the two contact bodies,
to be sure that a real contact area exists between them. The value of the force F_{c} is extremely important for the proper
operation of any contact system. It causes the contact solid surfaces to smash and to form one or several tiny contact platforms
(contact spots), as shown schematically in figure:
To pass from body 1 to body 2, the current must go through the contact spot, thus the current lines become bended in the contact region.
Due to the lengthening of these lines, the resistance in the contact region increases. But this is only the first main reason to introduce a special
kind of resistance  the contact resistance R_{c}, The second reason is that the contact surfaces are always contaminated
(gas molecules, dust, products of chemical reactions as oxides etc.).
Let us take a metal rod and let choose two points a and b along its axis. Let the measured resistance between these two points is R_{ab}. Then, we cut the rod in the middle between a and b, press the two pieces 1 and 2 together and measure R_{ab} again. Its new value R'_{ab} shall always be greater than R_{ab}: R'_{ab} = R_{ab} +R_{c} .The complementary component R_{c} > 0 is actually defined as contact resistance. One of its 2 subcomponents is based and depends on the current lines bending, while the second one depends on the contact surfaces contamination. Obviously the greater the contact spot area is, the less the bending. So, it can be concluded that when the contact force F_{c} increases, due to the more intensive smashing of the materials, R_{c} will decrease as depicted in the diagram:
This fact is experimentally proved. The reverse branch of the curve R_{c} = f(F_{c}), when F_{c} is reduced after reaching a maximum value F_{cmax},
is shown below. This is explained by the fact that materials cannot rebuild the structure prior to the pressing.
A formula has been introduced, reflecting a large number of obtained and processed experimental data.
Here K depends on the contact material, while m is associated with the type of the contact established:
m = 0,5 for a point contact, m = 0,70,8 for a linear contact and m = 1 for a plain contact.
Some data for the coefficient K is presented in table:

Electrical contacts  models and experiments 
MODEL 1
MODEL 2 MODEL 3 MODEL 4 MODEL 5 MODEL 5 
EXPERIMENT contact 1 EXPERIMENT contact 2 EXPERIMENT contact 3 EXPERIMENT contact 4 GALLERY AC/DC Module. Modeling, Analysis and Design.





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