Electricity is perhaps the most studied source of energy by mankind and, as a result, has been introduced into our lives so tightly that even to think that it is not there is blasphemy. But once, for many centuries, people used only a funny toy in everyday life - the attraction of small objects with rubbed wool. This stone was the current amber, and in Greek it was called "electron." After many centuries, this name was given to the first discovered elementary particle - the lightest of those carrying charge. Fate was favorable to the electron: it was he who became the main carrier of the energy of the electric field.
At its initial level, matter represents an atomic structure of a planetary type - in the center is a nucleus of proton neutrons and electron shells. The outer electronic shells of atoms of matter exchange electrons, therefore, in the interatomic space of some of them, mainly those that exhibit the properties of metals, a cloud of free electrons is formed. Usually, wires for transmitting electric current are made from such materials. The efficiency of current transmission is influenced by many factors, including and conductor resistance is an integral feature of every conductive material. If an electric voltage source is connected to the ends of the conductor, an electric current will flow through it . The reason for its occurrence is the potential difference at the ends of the conductor. Once in an electric field, electrons form directed motion from a larger potential to a smaller one, along a conductor of any length, and at the same time transport the energy of the power source. Any consumer connected to an electric circuit can use this energy.
The best of the materials-conductors are used for the manufacture of wires of trunk lines that transmit electric current thousands of kilometers from power plants. What serves as a criterion for the selection of material conductors? Such a characteristic is the resistance of the conductor. How does it appear in a conductor? According to the theory of electricity, free electrons move along a conductor with a certain energy. Collisions with atoms of matter arise on the path of their motion, and you have to share energy with them. We feel this “redistribution”, and in fact, energy loss, like heating a conductor.
Hence the eternal engineering problems in which the main participant in the action is the resistance of the conductor. It is this parameter of the electric circuit that determines the irretrievable energy loss, and they proportionally increase with increasing length of the wire L. The next geometric parameter of the conductor, affecting the resistance, is the cross section of the wire S. With increasing cross-section of the conductor, its resistance decreases proportionally. To assess the materials from the point of view of their suitability for use as a wire, another characteristic is used, which is called "resistivity": this is the resistance of a conductor with a cross section of 1 mm square. and a length of 1 m. Now, having previously taken from the table the value of resistivity ƿ for the corresponding material, we can calculate the resistance of the conductor. The formula gives the value R - Ohm, if ƿ - Ohm * m / mm sq. , S - mm sq., L - m.
R = (ƿ * L) / S.
Using this formula, you can perform calculations for any cases if the source data are known. But what if there is a conductor, but there are no tables and measuring instruments for the diameter and length of the wire at hand, in other words, what instrument measures the resistance of the conductor? In such cases, a measuring device called an ohmmeter is used.
There are many different options for ohmmeters that implement all kinds of operating principles, but most often they use current measuring circuits through the test resistance of the conductor and a calibrated resistor or measure the voltage drop across the test resistor with a calibrated current in the measuring circuit.