Table of resistivity of metals. What is electrical resistivity

Electric current occurs as a result of closing a circuit with a potential difference across the terminals. Field forces act on free electrons and they move along the conductor. During this journey, electrons meet atoms and transfer some of their accumulated energy to them. As a result, their speed decreases. But, due to the impact electric field, it is gaining momentum again. Thus, electrons constantly experience resistance, which is why the electric current heats up.

The property of a substance to convert electricity into heat when exposed to current is electrical resistance and is denoted as R, its measuring unit is Ohm. The amount of resistance depends mainly on the ability various materials conduct current.
For the first time, the German researcher G. Ohm spoke about resistance.

In order to find out the dependence of current on resistance, the famous physicist conducted many experiments. For experiments he used various conductors and received various indicators.
The first thing that G. Ohm determined was that the resistivity depends on the length of the conductor. That is, if the length of the conductor increased, the resistance also increased. As a result, this relationship was determined to be directly proportional.

The second relationship is the cross-sectional area. It could be determined by cross-sectioning the conductor. The area of the figure formed on the cut is the cross-sectional area. Here the relationship is inversely proportional. That is, the larger the cross-sectional area, the lower the conductor resistance became.

And the third, important quantity on which resistance depends is the material. As a result of the fact that Ohm used different materials in his experiments, he discovered different resistance properties. All these experiments and indicators were summarized in a table from which it can be seen different meaning specific resistance of various substances.

It is known that the best conductors are metals. Which metals are the best conductors? The table shows that copper and silver have the least resistance. Copper is used more often due to its lower cost, and silver is used in the most important and critical devices.

Substances with high resistivity in the table do not conduct electricity well, which means they can be excellent insulating materials. Substances that have this property to the greatest extent are porcelain and ebonite.

In general, specific electrical resistance is very important factor, after all, by determining its indicator, we can find out what substance the conductor is made of. To do this, you need to measure the cross-sectional area, find out the current using a voltmeter and ammeter, and also measure the voltage. This way we know the meaning resistivity and, using the table, we can easily find the substance. It turns out that resistivity is like a fingerprint of a substance. In addition, resistivity is important when planning long electrical circuits: we need to know this indicator in order to maintain a balance between length and area.

There is a formula that determines that resistance is 1 ohm if, at a voltage of 1V, its current is 1A. That is, the resistance of a unit area and a unit length made of a certain substance is the specific resistance.

It should also be noted that the resistivity indicator directly depends on the frequency of the substance. That is, whether it has impurities. However, adding just one percent of manganese increases the resistance of the most conductive substance, copper, by three times.

This table shows the electrical resistivity of some substances.

Highly conductive materials

Copper
As we have already said, copper is most often used as a conductor. This is explained not only by its low resistance. Copper has the advantages of high strength, corrosion resistance, ease of use and good machinability. Good brands copper is considered M0 and M1. The amount of impurities in them does not exceed 0.1%.

The high cost of the metal and its predominance in Lately scarcity encourages manufacturers to use aluminum as a conductor. Also, alloys of copper with various metals are used.
Aluminum
This metal is much lighter than copper, but aluminum has large values heat capacity and melting point. In this regard, in order to bring it to a molten state, more energy is required than copper. However, the fact of copper deficiency must be taken into account.
In the production of electrical products, as a rule, A1 grade aluminum is used. It contains no more than 0.5% impurities. And metal highest frequency- this is aluminum grade AB0000.
Iron
The cheapness and availability of iron is overshadowed by its high resistivity. In addition, it corrodes quickly. For this reason, steel conductors are often coated with zinc. The so-called bimetal is widely used - this is steel coated with copper for protection.
Sodium
Sodium is also an accessible and promising material, but its resistance is almost three times that of copper. In addition, metallic sodium has high chemical activity, which requires covering such a conductor with hermetically sealed protection. It should also protect the conductor from mechanical damage, since sodium is a very soft and rather fragile material.

Superconductivity
The table below shows the resistivity of substances at a temperature of 20 degrees. The indication of temperature is not accidental, because resistivity directly depends on this indicator. This is explained by the fact that when heated, the speed of atoms also increases, which means the probability of them meeting electrons will also increase.

It is interesting what happens to resistance under cooling conditions. For the first time, the behavior of atoms at very low temperatures noted by G. Kamerlingh Onnes in 1911. He cooled the mercury wire to 4K and found that its resistance dropped to zero. The change in the resistivity index of some alloys and metals under low temperature conditions is called superconductivity by the physicist.

Superconductors go into a state of superconductivity when cooled, and their optical and structural characteristics do not change. The main discovery is that electrical and magnetic properties metals in a superconducting state are very different from their properties in the normal state, as well as from the properties of other metals that cannot transition to this state when the temperature decreases.
The use of superconductors is carried out mainly in obtaining super-strong magnetic field, the force of which reaches 107 A/m. Superconducting power line systems are also being developed.

Similar materials.

Constantan (58.8 Cu, 40 Ni, 1.2 Mn)
Manganin (85 Cu, 12 Mn, 3 Ni)
Nickel silver (65 Cu, 20 Zn, 15 Ni)
Nickelin (54 Cu, 20 Zn, 26 Ni)
Nichrome (67.5 Ni, 15 Cr, 16 Fe, 1.5 Mn)
Rheonate (84Cu, 12Mn, 4 Zn)
Fechral (80 Fe, 14 Cr, 6 Al)

Nichrome resistivity

Every body through which an electric current is passed automatically exhibits a certain resistance to it. The property of a conductor to resist electric current is called electrical resistance.

Let's consider the electronic theory of this phenomenon. When moving along a conductor, free electrons constantly encounter other electrons and atoms on their way. By interacting with them, a free electron loses part of its charge. Thus, the electrons encounter resistance from the conductor material. Each body has its own atomic structure, which provides different resistance to electric current. The unit of resistance is considered to be Ohm. The resistance of materials is designated R or r.

The lower the resistance of the conductor, the easier it is for electric current to pass through this body. And vice versa: the higher the resistance, the worse the body conducts electric current.

The resistance of each individual conductor depends on the properties of the material from which it is made. To accurately characterize the electrical resistance of a particular material, the concept of resistivity (nichrome, aluminum, etc.) was introduced. Specific resistance is considered to be the resistance of a conductor up to 1 m long, the cross-section of which is 1 square meter. mm. This indicator is denoted by the letter p. Each material used in the production of a conductor has its own resistivity. For example, consider the resistivity of nichrome and fechral (more than 3 mm):

Х15Н60 — 1.13 Ohm*mm/m
Х23У5Т — 1.39 Ohm*mm/m
Х20Н80 — 1.12 Ohm*mm/m
ХН70У — 1.30 Ohm*mm/m
ХН20УС — 1.02 Ohm*mm/m

The resistivity of nichrome and fechral indicates their main area of application: the manufacture of devices thermal action, household appliances and electrical heating elements industrial furnaces.

Since nichrome and fechral are mainly used in the production of heating elements, the most common products are nichrome thread, tape, strip X15N60 and X20N80, as well as fechral wire X23Yu5T.

When closed electrical circuit, at the terminals of which there is a potential difference, an electric current arises. Free electrons, under the influence of electric field forces, move along the conductor. In their movement, electrons collide with the atoms of the conductor and give them a supply of their kinetic energy. The speed of electron movement continuously changes: when electrons collide with atoms, molecules and other electrons, it decreases, then under the influence of an electric field it increases and decreases again during a new collision. As a result, the conductor is installed uniform motion flow of electrons at a speed of several fractions of a centimeter per second. Consequently, electrons passing through a conductor always encounter resistance to their movement from its side. When passing electric current through the conductor the latter is heated.

Electrical resistance

The electrical resistance of a conductor, which is designated Latin letter r, is the property of a body or medium to transform electrical energy into heat when an electric current passes through it.

In the diagrams, electrical resistance is indicated as shown in Figure 1, A.

Variable electrical resistance, which serves to change the current in a circuit, is called rheostat. In the diagrams, rheostats are designated as shown in Figure 1, b. IN general view A rheostat is made from a wire of one resistance or another, wound on an insulating base. The slider or rheostat lever is placed in a certain position, as a result of which the required resistance is introduced into the circuit.

A long conductor with a small cross-section creates a large resistance to current. Short conductors with a large cross-section offer little resistance to current.

If we take two conductors from different materials, but the same length and cross-section, then the conductors will conduct current differently. This shows that the resistance of a conductor depends on the material of the conductor itself.

The temperature of the conductor also affects its resistance. As temperature increases, the resistance of metals increases, and the resistance of liquids and coal decreases. Only some special metal alloys (manganin, constantan, nickel and others) hardly change their resistance with increasing temperature.

So, we see that the electrical resistance of a conductor depends on: 1) the length of the conductor, 2) the cross-section of the conductor, 3) the material of the conductor, 4) the temperature of the conductor.

The unit of resistance is one ohm. Om is often represented by the Greek capital letter Ω (omega). Therefore, instead of writing “The conductor resistance is 15 ohms,” you can simply write: r= 15 Ω.
1,000 ohms is called 1 kiloohms(1kOhm, or 1kΩ),
1,000,000 ohms is called 1 megaohm(1mOhm, or 1MΩ).

When comparing the resistance of conductors from different materials, it is necessary to take a certain length and cross-section for each sample. Then we will be able to judge which material conducts electric current better or worse.

Video 1. Conductor resistance

Electrical resistivity

The resistance in ohms of a conductor 1 m long, with a cross section of 1 mm² is called resistivity and is designated Greek letter ρ (ro).

Table 1 shows the resistivities of some conductors.

Table 1

Resistivities of various conductors

The table shows that an iron wire with a length of 1 m and a cross-section of 1 mm² has a resistance of 0.13 Ohm. To get 1 Ohm of resistance you need to take 7.7 m of such wire. Silver has the lowest resistivity. 1 Ohm of resistance can be obtained by taking 62.5 m of silver wire with a cross section of 1 mm². Silver is the best conductor, but the cost of silver excludes the possibility of its mass use. After silver in the table comes copper: 1 m of copper wire with a cross section of 1 mm² has a resistance of 0.0175 Ohm. To get a resistance of 1 ohm, you need to take 57 m of such wire.

Chemically pure copper, obtained by refining, has found widespread use in electrical engineering for the manufacture of wires, cables, windings of electrical machines and devices. Aluminum and iron are also widely used as conductors.

The conductor resistance can be determined by the formula:

Where r– conductor resistance in ohms; ρ – specific resistance of the conductor; l– length of the conductor in m; S– conductor cross-section in mm².

Example 1. Determine the resistance of 200 m of iron wire with a cross section of 5 mm².

Example 2. Calculate the resistance of 2 km of aluminum wire with a cross section of 2.5 mm².

From the resistance formula you can easily determine the length, resistivity and cross-section of the conductor.

Example 3. For a radio receiver, it is necessary to wind a 30 Ohm resistance from nickel wire with a cross section of 0.21 mm². Determine the required wire length.

Example 4. Determine the cross-section of 20 m of nichrome wire if its resistance is 25 Ohms.

Example 5. A wire with a cross section of 0.5 mm² and a length of 40 m has a resistance of 16 Ohms. Determine the wire material.

The material of the conductor characterizes its resistivity.

Based on the resistivity table, we find that lead has this resistance.

It was stated above that the resistance of conductors depends on temperature. Let's do the following experiment. Let's wind several meters of thin metal wire in the form of a spiral and connect this spiral to the battery circuit. To measure current, we connect an ammeter to the circuit. When the coil is heated in the burner flame, you will notice that the ammeter readings will decrease. This shows that the resistance of a metal wire increases with heating.

For some metals, when heated by 100°, the resistance increases by 40–50%. There are alloys that change their resistance slightly with heating. Some special alloys show virtually no change in resistance when temperature changes. The resistance of metal conductors increases with increasing temperature, while the resistance of electrolytes (liquid conductors), coal and some solids, on the contrary, decreases.

The ability of metals to change their resistance with changes in temperature is used to construct resistance thermometers. This thermometer consists of a platinum wire wound on a mica frame. By placing a thermometer, for example, in a furnace and measuring the resistance of the platinum wire before and after heating, the temperature in the furnace can be determined.

The change in the resistance of a conductor when it is heated per 1 ohm of initial resistance and per 1° temperature is called temperature coefficient of resistance and is denoted by the letter α.

If at temperature t 0 conductor resistance is r 0 , and at temperature t equals r t, then the temperature coefficient of resistance

Note. Calculation using this formula can only be done in a certain temperature range (up to approximately 200°C).

We present the values of the temperature coefficient of resistance α for some metals (Table 2).

table 2

Temperature coefficient values for some metals

From the formula for the temperature coefficient of resistance we determine r t:

r t = r 0 .

Example 6. Determine the resistance of an iron wire heated to 200°C if its resistance at 0°C was 100 Ohms.

r t = r 0 = 100 (1 + 0.0066 × 200) = 232 ohms.

Example 7. A resistance thermometer made of platinum wire had a resistance of 20 ohms in a room at 15°C. The thermometer was placed in the oven and after some time its resistance was measured. It turned out to be equal to 29.6 Ohms. Determine the temperature in the oven.

Electrical conductivity

So far, we have considered the resistance of a conductor as the obstacle that the conductor provides to the electric current. But still, current flows through the conductor. Therefore, in addition to resistance (obstacle), the conductor also has the ability to conduct electric current, that is, conductivity.

The greater the resistance of a conductor, the less conductivity it has, the worse it conducts electric current, and, conversely, the lower the resistance of a conductor, the greater conductivity it has, the easier it is for current to pass through the conductor. Therefore, the resistance and conductivity of a conductor are reciprocal quantities.

From mathematics it is known that the inverse of 5 is 1/5 and, conversely, the inverse of 1/7 is 7. Therefore, if the resistance of a conductor is denoted by the letter r, then the conductivity is defined as 1/ r. Conductivity is usually denoted by the letter g.

Electrical conductivity is measured in (1/Ohm) or in siemens.

Example 8. The conductor resistance is 20 ohms. Determine its conductivity.

If r= 20 Ohm, then

Example 9. The conductivity of the conductor is 0.1 (1/Ohm). Determine its resistance

If g = 0.1 (1/Ohm), then r= 1 / 0.1 = 10 (Ohm)

Every substance is capable of conducting current to varying degrees, this value is affected by the resistance of the material. The resistivity of copper, aluminum, steel and any other element is indicated by the letter Greek alphabetρ. This value does not depend on such characteristics of the conductor as size, shape and physical state, ordinary electrical resistance takes these parameters into account. Resistivity is measured in Ohms multiplied by mm² and divided by meter.

Categories and their descriptions

Any material is capable of exhibiting two types of resistance depending on the electricity supplied to it. The current can be variable or constant, which significantly affects the technical performance of the substance. So, there are such resistances:

Ohmic. Appears under the influence of direct current. Characterizes friction, which is created by the movement of electrically charged particles in a conductor.
Active. It is determined according to the same principle, but is created under the influence of alternating current.

In this regard, there are also two definitions of specific value. For direct current, it is equal to the resistance exerted by a unit length of conductive material of a unit fixed cross-sectional area. The potential electric field affects all conductors, as well as semiconductors and solutions capable of conducting ions. This value determines the conductive properties of the material itself. The shape of the conductor and its dimensions are not taken into account, so it can be called basic in electrical engineering and materials science.

Under the condition of passing alternating current, the specific value is calculated taking into account the thickness of the conductive material. Here the influence of not only potential, but also eddy current occurs, and in addition, the frequency of electric fields is taken into account. The resistivity of this type is greater than with direct current, since here the positive value of the resistance to the vortex field is taken into account. This value also depends on the shape and size of the conductor itself. It is these parameters that determine the nature of the vortex motion of charged particles.

Alternating current causes certain electromagnetic phenomena in conductors. They are very important for the electrical characteristics of the conductive material:

The skin effect is characterized by a weakening of the electromagnetic field, the more it penetrates into the medium of the conductor. This phenomenon is also called the surface effect.
The proximity effect reduces current density due to the proximity of adjacent wires and their influence.

These effects are very important when calculating the optimal thickness of the conductor, since when using a wire whose radius is greater than the depth of current penetration into the material, the rest of its mass will remain unused, and therefore this approach will be ineffective. In accordance with the calculations carried out, the effective diameter of the conductive material in some situations will be as follows:

for a current of 50 Hz - 2.8 mm;
400 Hz - 1 mm;
40 kHz - 0.1 mm.

In view of this, the use of flat multicore cables, consisting of many thin wires, is actively used for high-frequency currents.

Characteristics of metals

Specific indicators of metal conductors are contained in special tables. Using these data, you can make the necessary further calculations. An example of such a resistivity table can be seen in the image.

The table shows that silver has the greatest conductivity - it is an ideal conductor among all existing metals and alloys. If you calculate how much wire from this material is required to obtain a resistance of 1 ohm, you will get 62.5 m. Iron wire for the same value will require as much as 7.7 m.

No matter how wonderful properties silver has, it is too expensive a material for mass use in electrical networks, therefore wide application I found copper in everyday life and industry. In terms of specific indicator, it is in second place after silver, and in terms of prevalence and ease of extraction, it is much better than it. Copper has other advantages that have allowed it to become the most common conductor. These include:

For use in electrical engineering, refined copper is used, which, after smelting from sulfide ore, goes through the processes of roasting and blowing, and then necessarily undergoes electrolytic purification. After such processing, it is possible to obtain a material that is very High Quality(grades M1 and M0), which will contain from 0.1 to 0.05% impurities. An important nuance is the presence of oxygen in extremely small quantities, since it negatively affects the mechanical characteristics of copper.

Often this metal is replaced by cheaper materials - aluminum and iron, as well as various bronzes (alloys with silicon, beryllium, magnesium, tin, cadmium, chromium and phosphorus). Such compositions have higher strength compared to pure copper, although they have lower conductivity.

Advantages of aluminum

Although aluminum has greater resistance and is more fragile, its widespread use is due to the fact that it is not as scarce as copper and therefore costs less. Aluminum has a resistivity of 0.028 and its low density makes it 3.5 times lighter than copper.

For electrical work, purified aluminum grade A1 is used, containing no more than 0.5% impurities. The higher grade AB00 is used for the manufacture of electrolytic capacitors, electrodes and aluminum foil. The impurity content in this aluminum is no more than 0.03%. There is also pure metal AB0000, including no more than 0.004% additives. The impurities themselves also matter: nickel, silicon and zinc have a slight effect on the conductivity of aluminum, and the content of copper, silver and magnesium in this metal has a noticeable effect. Thallium and manganese reduce conductivity the most.

Aluminum has good anti-corrosion properties. Upon contact with air, it becomes covered with a thin film of oxide, which protects it from further destruction. For improvement mechanical characteristics the metal is alloyed with other elements.

Indicators of steel and iron

The resistivity of iron compared to copper and aluminum is very high, however, due to its availability, strength and resistance to deformation, the material is widely used in electrical production.

Although iron and steel, whose resistivity is even higher, have significant disadvantages, manufacturers of conductor materials have found methods to compensate for them. In particular, low corrosion resistance is overcome by coating the steel wire with zinc or copper.

Properties of sodium

Sodium metal is also very promising in conductor production. In terms of resistance, it significantly exceeds copper, but has a density 9 times less than that. This allows the material to be used in the manufacture of ultra-light wires.

Sodium metal is very soft and completely unstable to any kind of deformation, which makes its use problematic - a wire made of this metal must be covered with a very strong sheath with extremely little flexibility. The shell must be airtight, since sodium exhibits strong chemical activity under the most neutral conditions. It instantly oxidizes in air and exhibits violent reaction with water, including that contained in the air.

Another benefit of using sodium is its availability. It can be obtained through the electrolysis of molten sodium chloride, of which there is an unlimited amount in the world. Other metals are clearly inferior in this regard.

To calculate the performance of a specific conductor, it is necessary to divide the product of the specific number and length of the wire by its cross-sectional area. The result will be the resistance value in Ohms. For example, to determine the resistance of 200 m of iron wire with a nominal cross-section of 5 mm², you need to multiply 0.13 by 200 and divide the result by 5. The answer is 5.2 Ohms.

Rules and features of calculation

Microohmmeters are used to measure the resistance of metallic media. Today they are produced in a digital version, so the measurements taken with their help are accurate. It can be explained by the fact that metals have high level conductivity and have extremely low resistance. For example, the lower threshold of measuring instruments has a value of 10 -7 Ohms.

Using microohmmeters, you can quickly determine how good the contact is and what resistance is exhibited by the windings of generators, electric motors and transformers, as well as electrical buses. It is possible to calculate the presence of inclusions of another metal in the ingot. For example, a piece of tungsten plated with gold exhibits half the conductivity of all gold. The same method can be used to determine internal defects and cavities in the conductor.

The resistivity formula is as follows: ρ = Ohm mm 2 /m. In words it can be described as the resistance of 1 meter of conductor, having a cross-sectional area of 1 mm². The temperature is assumed to be standard - 20 °C.

Effect of temperature on measurement

Heating or cooling of some conductors has a significant effect on the performance of measuring instruments. An example is the following experiment: it is necessary to connect a spirally wound wire to the battery and connect an ammeter to the circuit.

The more the conductor heats up, the lower the readings on the device become. Current strength is inversely proportional to resistance. Therefore, we can conclude that as a result of heating, the conductivity of the metal decreases. To a greater or lesser extent, all metals behave this way, but in some alloys there is practically no change in conductivity.

It is noteworthy that liquid conductors and some solid nonmetals tend to decrease their resistance as temperature increases. But scientists have also turned this ability of metals to their advantage. Knowing the temperature coefficient of resistance (α) when heating some materials, it is possible to determine the external temperature. For example, a platinum wire placed on a mica frame is placed in a furnace and the resistance is measured. Depending on how much it has changed, a conclusion is drawn about the temperature in the oven. This design is called a resistance thermometer.

If at temperature t 0 conductor resistance is r 0, and at temperature t equals rt, then the temperature coefficient of resistance is equal to

Calculation using this formula can only be done in a certain temperature range (up to approximately 200 °C).

Electric current I in any substance is created by the movement of charged particles in a certain direction due to the application of external energy (potential difference U). Each substance has individual properties that differently affect the passage of current in it. These properties are assessed by electrical resistance R.

Georg Ohm empirically determined the factors influencing the electrical resistance of a substance and derived it from voltage and current, which is named after him. Resistance unit in international system SI is named after him. 1 Ohm is the resistance value measured at a temperature of 0 ° C for a homogeneous mercury 106.3 cm long with a cross-sectional area of 1 mm 2.

Definition

To evaluate and put into practice materials for the manufacture of electrical devices, the term "conductor resistivity". The added adjective “specific” indicates the factor of using the reference volume value adopted for the substance in question. This makes it possible to evaluate the electrical parameters of different materials.

It is taken into account that the resistance of the conductor increases with increasing its length and decreasing cross-section. The SI system uses a volume of a homogeneous conductor with a length of 1 meter and a cross-section of 1 m 2. In technical calculations, an outdated but convenient non-system unit of volume is used, consisting of a length of 1 meter and an area of 1 mm 2. The formula for resistivity ρ is shown in the figure.

To determine the electrical properties of substances, another characteristic was introduced - specific conductivity b. It is inversely proportional to the resistivity value and determines the ability of the material to conduct electric current: b = 1/ρ.

How does resistivity depend on temperature?

The conductivity of a material is affected by its temperature. Different groups of substances behave differently when heated or cooled. This property is taken into account in electrical wires operating outdoors in hot and cold weather.

The material and resistivity of the wire are selected taking into account the operating conditions.

The increase in the resistance of conductors to the passage of current when heated is explained by the fact that as the temperature of the metal increases, the intensity of movement of atoms and carriers in it increases electric charges in all directions, which creates unnecessary obstacles to the movement of charged particles in one direction and reduces the magnitude of their flow.

If you reduce the temperature of the metal, the conditions for the passage of current improve. When cooled to a critical temperature, many metals exhibit the phenomenon of superconductivity, when their electrical resistance is practically zero. This property is widely used in powerful electromagnets.

The effect of temperature on the conductivity of metal is used by the electrical industry in the manufacture of ordinary incandescent lamps. When a current passes through them, it heats up to such a state that it emits a luminous flux. Under normal conditions, the resistivity of nichrome is about 1.05÷1.4 (ohm ∙mm 2)/m.

When the light bulb is turned on, a large current passes through the filament, which very quickly heats up the metal. At the same time, the resistance of the electrical circuit increases, limiting the initial current to the nominal value required to obtain lighting. In this way, the current strength is easily regulated through a nichrome spiral, eliminating the need to use complex ballasts used in LED and fluorescent sources.

How is the resistivity of materials used in technology?

Non-ferrous precious metals have best properties electrical conductivity. Therefore, critical contacts in electrical devices are made of silver. But this increases the final cost of the entire product. The most acceptable option is to use cheaper metals. For example, the resistivity of copper equal to 0.0175 (ohm ∙mm 2)/m is quite suitable for such purposes.

Noble metals- gold, silver, platinum, palladium, iridium, rhodium, ruthenium and osmium, named mainly due to their high chemical resistance and beautiful appearance in jewelry. In addition, gold, silver and platinum have high ductility, and metals platinum group- refractoriness and, like gold, chemical inertness. These advantages noble metals combine.

Copper alloys, which have good conductivity, are used to make shunts that limit the flow of large currents through the measuring head of high-power ammeters.

The resistivity of aluminum 0.026÷0.029 (ohm ∙mm 2)/m is slightly higher than that of copper, but the production and cost of this metal is lower. Plus it's lighter. This explains its widespread use in the energy sector for the manufacture of outdoor wires and cable cores.

The resistivity of iron 0.13 (ohm ∙mm 2)/m also allows its use for transmitting electric current, but this results in greater power losses. Steel alloys have increased strength. Therefore, steel threads are woven into the aluminum overhead wires of high-voltage power lines, which are designed to withstand tensile loads.

This is especially true when ice forms on wires or strong gusts of wind.

Some alloys, for example, constantine and nickel, have thermally stable resistive characteristics in a certain range. Nickel's electrical resistivity remains virtually unchanged from 0 to 100 degrees Celsius. Therefore, spirals for rheostats are made of nickel.

IN measuring instruments The property of strictly changing the resistivity values of platinum depending on its temperature is widely used. If electric current from a stabilized voltage source is passed through a platinum conductor and the resistance value is calculated, it will indicate the temperature of the platinum. This allows the scale to be graduated in degrees corresponding to Ohm values. This method allows you to measure temperature with an accuracy of fractions of degrees.

Sometimes to solve practical problems you need to know cable impedance or specific resistance. For this purpose, reference books for cable products provide the values of inductive and active resistance of one core for each cross-sectional value. With their help, permissible loads and heat generated are calculated, acceptable operating conditions are determined and effective protection is selected.

On conductivity metals are influenced by the way they are processed. Using pressure to cause plastic deformation breaks the structure crystal lattice, increases the number of defects and increases resistance. To reduce it, recrystallization annealing is used.

Stretching or compressing metals causes elastic deformation in them, from which the amplitudes of thermal vibrations of electrons decrease and the resistance decreases somewhat.

When designing grounding systems, it is necessary to take into account. It differs in definition from the above method and is measured in SI units - Ohm∙meter. It is used to evaluate the quality of the flow of electric current inside the earth.

The conductivity of soil is influenced by many factors, including soil moisture, density, particle size, temperature, and the concentration of salts, acids and alkalis.