Temperature coefficients of resistance of metals. Specific resistances and temperature coefficient of resistance of metals and alloys

Conductor resistance (R) (resistivity) () depends on temperature. This dependence for minor changes in temperature () is presented as a function:

where is the resistivity of the conductor at a temperature of 0 o C; — temperature coefficient resistance.

DEFINITION

Temperature coefficient of electrical resistance() are called physical quantity, equal to the relative increment (R) of the circuit section (or resistivity of the medium ()), which occurs when the conductor is heated by 1 o C. Mathematically, the determination of the temperature coefficient of resistance can be represented as:

The value characterizes the relationship between electrical resistance and temperature.

At temperatures within the range, for most metals the coefficient under consideration remains constant. For pure metals, the temperature coefficient of resistance is often taken to be

Sometimes they talk about the average temperature coefficient of resistance, defining it as:

where is the average value of the temperature coefficient in a given temperature range ().

Temperature coefficient of resistance for different substances

Most metals have a temperature coefficient of resistance greater than zero. This means that the resistance of metals increases with increasing temperature. This occurs as a result of electron scattering on the crystal lattice, which enhances thermal vibrations.

At temperatures close to absolute zero (-273 o C), the resistance of a large number of metals sharply drops to zero. Metals are said to go into a superconducting state.

Semiconductors that do not have impurities have a negative temperature coefficient of resistance. Their resistance decreases with increasing temperature. This occurs due to the fact that the number of electrons that move into the conduction band increases, which means that the number of holes per unit volume of the semiconductor increases.

Electrolyte solutions have . The resistance of electrolytes decreases with increasing temperature. This occurs because the increase in the number of free ions as a result of the dissociation of molecules exceeds the increase in the scattering of ions as a result of collisions with solvent molecules. It must be said that the temperature coefficient of resistance for electrolytes is a constant value only in a small temperature range.

Units

The basic SI unit for measuring the temperature coefficient of resistance is:

Examples of problem solving

EXAMPLE 1

Exercise	An incandescent lamp with a tungsten spiral is connected to a network with voltage B, current A flows through it. What will be the temperature of the spiral if at a temperature o C it has a resistance Ohm? Temperature coefficient of resistance of tungsten .
Solution	As a basis for solving the problem, we use the formula for the dependence of resistance on temperature of the form: where is the resistance of the tungsten filament at a temperature of 0 o C. Expressing from expression (1.1), we have: According to Ohm's law, for a section of the circuit we have: Let's calculate Let's write the equation connecting resistance and temperature: Let's carry out the calculations:
Answer	K

EXAMPLE 2

Exercise

At temperature, the resistance of the rheostat is equal to , the resistance of the ammeter is equal and it shows the current strength. The rheostat is made of iron wire, it is connected in series with the ammeter (Fig. 1). How much current will flow through the ammeter if the rheostat is heated to a temperature? Consider the temperature coefficient of resistance of iron to be equal to .

Metal	Specific resistance ρ at 20 ºС, Ohm*mm²/m	Temperature coefficient of resistance α, ºС -1
Aluminum
Iron (steel)
Constantan
Manganin

The temperature coefficient of resistance α shows how much the resistance of a conductor of 1 ohm increases with an increase in temperature (heating of the conductor) by 1 ºС.

The conductor resistance at temperature t is calculated by the formula:

r t = r 20 + α* r 20 *(t - 20 ºС)

where r 20 is the resistance of the conductor at a temperature of 20 ºС, r t is the resistance of the conductor at temperature t.

Current Density

A current I = 10 A flows through a copper conductor with a cross-sectional area S = 4 mm². What is the current density?

Current density J = I/S = 10 A/4 mm² = 2.5 A/mm².

[A current I = 2.5 A flows through a cross-sectional area of ​​1 mm²; a current I = 10 A flows throughout the entire cross section S].

A switchgear bus of rectangular cross-section (20x80) mm² carries a current I = 1000 A. What is the current density in the bus?

Cross-sectional area of ​​the tire S = 20x80 = 1600 mm². Current Density

J = I/S = 1000 A/1600 mm² = 0.625 A/mm².

The coil's wire has a circular cross-section with a diameter of 0.8 mm and allows a current density of 2.5 A/mm². What permissible current can be passed through the wire (heating should not exceed the permissible)?

Cross-sectional area of ​​the wire S = π * d²/4 = 3/14*0.8²/4 ≈ 0.5 mm².

Allowable current I = J*S = 2.5 A/mm² * 0.5 mm² = 1.25 A.

Permissible current density for the transformer winding J = 2.5 A/mm². A current I = 4 A passes through the winding. What should be the cross-section (diameter) of the circular cross-section of the conductor so that the winding does not overheat?

Cross-sectional area S = I/J = (4 A) / (2.5 A/mm²) = 1.6 mm²

This section corresponds to a wire diameter of 1.42 mm.

An insulated copper wire with a cross-section of 4 mm² carries a maximum permissible current of 38 A (see table). What is the permissible current density? What are the permissible current densities for copper wires with cross-sections of 1, 10 and 16 mm²?

1). Allowable current density

J = I/S = 38 A / 4mm² = 9.5 A/mm².

2). For a cross section of 1 mm², the permissible current density (see table)

J = I/S = 16 A / 1 mm² = 16 A/mm².

3). For a cross section of 10 mm² permissible current density

J = 70 A / 10 mm² = 7.0 A/mm²

4). For a cross section of 16 mm² permissible current density

J = I/S = 85 A / 16 mm² = 5.3 A/mm².

The permissible current density decreases with increasing cross-section. Table valid for electrical wires with class B insulation.

Problems to solve independently

A current I = 4 A should flow through the transformer winding. What should be the cross-section of the winding wire with an allowable current density of J = 2.5 A/mm²? (S = 1.6 mm²)

A wire with a diameter of 0.3 mm carries a current of 100 mA. What is the current density? (J = 1.415 A/mm²)

Along the winding of an electromagnet made of insulated wire with a diameter

d = 2.26 mm (excluding insulation) a current of 10 A passes. What is the density

current? (J = 2.5 A/mm²).

4. The transformer winding allows a current density of 2.5 A/mm². The current in the winding is 15 A. What is the smallest cross-section and diameter that a round wire can have (excluding insulation)? (in mm²; 2.76 mm).

Temperature coefficient of electrical resistance, TKS- a value or set of values ​​expressing the dependence of electrical resistance on temperature.

The dependence of resistance on temperature can be of a different nature, which can be expressed in the general case by some function. This function can be expressed through a dimensional constant, where is a certain specified temperature, and a dimensionless temperature-dependent coefficient of the form:

.

In this definition, it turns out that the coefficient depends only on the properties of the medium and does not depend on the absolute value of the resistance of the measured object (determined by its geometric dimensions).

If the temperature dependence (in a certain temperature range) is sufficiently smooth, it can be fairly well approximated by a polynomial of the form:

The coefficients at the powers of the polynomial are called temperature coefficients of resistance. Thus, the temperature dependence will have the form (for brevity we denote it as):

and, if we take into account that the coefficients depend only on the material, the resistivity can also be expressed:

The coefficients have the dimensions of Kelvin, or Celsius, or another temperature unit to the same extent, but with a minus sign. The temperature coefficient of resistance of the first degree characterizes linear dependence electrical resistance depends on temperature and is measured in kelvins to the minus first power (K⁻¹). The temperature coefficient of the second degree is quadratic and is measured in kelvins minus the second degree (K⁻²). The coefficients of higher degrees are expressed similarly.

So, for example, for a platinum temperature sensor of the Pt100 type, the method for calculating resistance looks like

that is, for temperatures above 0°C the coefficients are used α₁=3.9803·10⁻³ K⁻¹, α₂=−5.775·10⁻⁷ K⁻² at T₀=0°C (273.15 K), and for temperatures below 0°C, α₃=4.183·10⁻⁹ K⁻³ and α₄=−4.183·10⁻¹² K⁻⁴ are added.

Although several powers are used for accurate calculations, in most practical cases one linear coefficient is sufficient, and this is usually what is meant by TCS. Thus, for example, a positive TCR means an increase in resistance with increasing temperature, and a negative TCR means a decrease.

The main reasons for changes in electrical resistance are changes in the concentration of charge carriers in the medium and their mobility.

Materials with high TCR are used in temperature-sensitive circuits as part of thermistors and bridge circuits made from them. For precise temperature changes, thermistors based on

Temperature coefficient of resistance(α) - relative change in the resistance of a section of an electrical circuit or the electrical resistivity of a material when the temperature changes by 1, expressed in K -1. In electronics, resistors are used in particular from special metal alloys with low α values, such as manganin or constantan alloys and semiconductor components with large positive or negative α values ​​(thermistors). Physical meaning The temperature coefficient of resistance is expressed by the equation:

Where dR- change in electrical resistance R when the temperature changes by dT.

Conductors

The temperature dependence of resistance for most metals is close to linear over a wide temperature range and is described by the formula:

R T R0- electrical resistance at initial temperature T 0 [Ohm]; α - temperature coefficient of resistance; ΔT- temperature change is TT 0 [K].

At low temperatures The temperature dependence of the resistance of conductors is determined by Mathiesen's rule.

Semiconductors

Dependence of NTC thermistor resistance on temperature

For semiconductor devices such as thermistors, the temperature dependence of resistance is mainly determined by the dependence of charge carrier concentration on temperature. This is an exponential relationship:

R T- electrical resistance at temperature T [Ohm]; R∞- electrical resistance at temperature T = ∞ [Ohm]; W g- band gap - the range of energy values ​​that an electron does not have in an ideal (defect-free) crystal [eV]; k- Boltzmann constant [eV/K].

Taking logarithms of the left and right sides of the equation, we get:

, where is the material constant.

The temperature coefficient of resistance of a thermistor is determined by the equation:

From the dependence of R T on T we have:

Sources

• Theoretical basis electrical engineering: Textbook: 3 volumes / V. S. Boyko, V. V. Boyko, Yu. F. Vydolob et al.; Under general ed. I. M. Chizhenko, V. S. Boyko. - M.: ShTs "Publishing house" Polytechnic "", 2004. - T. 1: stable modes of linear electrical circuits with lumped parameters. - 272 p.: ill. ISBN 966-622-042-3
• Shegedin A.I. Painter V.S. Theoretical foundations of electrical engineering. Part 1: Tutorial for students of distance learning in electrical engineering and electromechanical specialties of higher education educational institutions. - M.: Magnolia Plus, 2004. - 168 p.
• I.M.Kucheruk, I.T.Gorbachuk, P.P.Lutsik (2006). General course physics: Textbook in 3 volumes. T.2. Electricity and magnetism. Kyiv: Technique.

The results of resistivity measurements are greatly influenced by shrinkage cavities, gas bubbles, inclusions and other defects. Moreover, Fig. 155 shows that small amounts of impurity entering the solid solution also have a large effect on the measured conductivity. Therefore, it is much more difficult to produce satisfactory samples for measuring electrical resistance than for

dilatometric study. This led to another method of constructing phase diagrams, in which the temperature coefficient of resistance is measured.

Temperature coefficient of resistance

Electrical resistance at temperature

Matthiessen found that the increase in metal resistance due to the presence of a small amount of the second component in the solid solution does not depend on temperature; it follows that for such a solid solution the value does not depend on the concentration. This means that the temperature coefficient of resistance is proportional, i.e., conductivity, and the graph of the coefficient a depending on the composition is similar to the graph of the conductivity of a solid solution. There are many known exceptions to this rule, especially for transition metals, but for most cases it is approximately true.

The temperature coefficient of resistance of intermediate phases is usually of the same order of magnitude as for pure metals, even in cases where the connection itself has high resistance. There are, however, intermediate phases whose temperature coefficient in a certain temperature range equal to zero or negative.

Matthiessen's rule applies, strictly speaking, only to solid solutions, but there are many cases where it appears to be true also for two-phase alloys. If the temperature coefficient of resistance is plotted against composition, the curve usually has the same shape as the conductivity curve, so the phase transformation can be detected in the same way. This method is convenient to use when, due to fragility or other reasons, it is impossible to produce samples suitable for conductivity measurements.

In practice, the average temperature coefficient between two temperatures is determined by measuring the electrical resistance of the alloy at those temperatures. If no phase transformation occurs in the temperature range under consideration, then the coefficient is determined by the formula:

will have the same meaning as if the interval is small. For hardened alloys as temperatures and

It is convenient to take 0° and 100°, respectively, and the measurements will give the phase region at the quenching temperature. However, if measurements are made at high temperatures, the interval should be much less than 100°, if the phase boundary may be somewhere between the temperatures

Rice. 158. (see scan) Electrical conductivity and temperature coefficient of electrical resistance in the silver-magic system (Tamman)

The great advantage of this method is that the coefficient a depends on the relative resistance of the sample at two temperatures, and is thus not affected by pitting and other metallurgical defects in the sample. Conductivity and temperature coefficient curves

resistances in some alloy systems repeat one another. Rice. 158 is taken from Tammann's early work (the curves refer to silver-magnesium alloys); later work showed that the region of the -solid solution decreases with decreasing temperature and a superstructure exists in the region of the phase. Some other phase boundaries in Lately have also undergone changes, so that the diagram presented in Fig. 158 is of historical interest only and cannot be used for accurate measurements.

Free electron concentration n in a metal conductor with increasing temperature remains practically unchanged, but their average speed of thermal movement increases. The vibrations of the crystal lattice nodes also increase. The quantum of elastic vibrations of the medium is usually called phonon. Small thermal vibrations of the crystal lattice can be considered as a collection of phonons. With increasing temperature, the amplitudes of thermal vibrations of atoms increase, i.e. the cross section of the spherical volume occupied by the vibrating atom increases.

Thus, with increasing temperature, more and more obstacles appear in the path of electron drift under the influence of electric field. This results in a decrease average length electron free path λ, electron mobility decreases and, as a result, the conductivity of metals decreases and the resistivity increases (Fig. 3.3). The change in the resistivity of a conductor when its temperature changes by 3K, related to the resistivity value of this conductor at a given temperature, is called the temperature coefficient of resistivity TK ρ or. The temperature coefficient of resistivity is measured in K -3. The temperature coefficient of resistivity of metals is positive. As follows from the definition given above, the differential expression for TK ρ has the form:

According to the conclusions of the electronic theory of metals, the values ​​of pure metals in the solid state should be close to the temperature coefficient (TK) of expansion ideal gases, i.e. 3: 273 =0.0037. In fact, most metals have ≈ 0.004. Some metals have higher values, including ferromagnetic metals - iron, nickel and cobalt.

Note that for each temperature there is a temperature coefficient TK ρ. In practice, for a certain temperature range, the average value is used TK ρ or:

Where ρ3 And ρ2- resistivity of conductor material at temperatures T3 And T2 respectively (in this case T2 > T3); there is a so-called average temperature coefficient of resistivity of this material in the temperature range from T3 before T2.

One of the most popular metals in industries is copper. It is most widely used in electrical and electronics. Most often it is used in the manufacture of windings for electric motors and transformers. The main reason for using this particular material is that copper has the lowest... currently materials with electrical resistivity. Until it appears new material with a lower value of this indicator, we can say with confidence that there will be no replacement for copper.

General characteristics of copper

Speaking about copper, it must be said that at the dawn of the electrical era it began to be used in the production of electrical equipment. They began to use it largely due to unique properties, which this alloy possesses. In itself it represents a material that differs high properties in terms of ductility and good malleability.

Along with the thermal conductivity of copper, one of its most important advantages is its high electrical conductivity. It is due to this property that copper and has become widespread in power plants, in which it acts as a universal conductor. The most valuable material is electrolytic copper, which has a high degree of purity of 99.95%. Thanks to this material, it becomes possible to produce cables.

Pros of using electrolytic copper

The use of electrolytic copper allows you to achieve the following:

• Ensure high electrical conductivity;
• Achieve excellent styling ability;
• Provide a high degree of plasticity.

Areas of application

Cable products made from electrolytic copper are widely used in various industries. Most often it is used in the following areas:

• electrical industry;
• electrical appliances;
• automotive industry;
• production of computer equipment.

What is the resistivity?

To understand what copper is and its characteristics, it is necessary to understand the main parameter of this metal - resistivity. It should be known and used when performing calculations.

Resistivity is usually understood as a physical quantity, which is characterized as the ability of a metal to conduct electric current.

It is also necessary to know this value in order to correctly calculate electrical resistance conductor. When making calculations, they are also guided by its geometric dimensions. When carrying out calculations, use the following formula:

This formula is familiar to many. Using it, you can easily calculate the resistance of a copper cable, focusing only on the characteristics of the electrical network. It allows you to calculate the power that is inefficiently spent on heating the cable core. Besides, a similar formula allows you to calculate resistance any cable. It does not matter what material was used to make the cable - copper, aluminum or some other alloy.

A parameter such as electrical resistivity is measured in Ohm*mm2/m. This indicator for copper wiring laid in an apartment is 0.0175 Ohm*mm2/m. If you try to look for an alternative to copper - a material that could be used instead, then only silver can be considered the only suitable one, whose resistivity is 0.016 Ohm*mm2/m. However, when choosing a material, it is necessary to pay attention not only to resistivity, but also to reverse conductivity. This value is measured in Siemens (Cm).

Siemens = 1/ Ohm.

For copper of any weight, this composition parameter is 58,100,000 S/m. As for silver, its reverse conductivity is 62,500,000 S/m.

In our world high technology when every home has a large number of electrical devices and installations, the value of such a material as copper is simply invaluable. This material used to make wiring, without which no room can do. If copper did not exist, then man would have to use wires from other available materials, for example, from aluminum. However, in this case one would have to face one problem. The thing is that this material has a much lower conductivity than copper conductors.

Resistivity

The use of materials with low electrical and thermal conductivity of any weight leads to large losses of electricity. A this affects power loss on the equipment used. Most experts call copper as the main material for making insulated wires. It is the main material from which individual elements of equipment powered by electric current are made.

• Boards installed in computers are equipped with etched copper traces.
• Copper is also used to make a wide variety of components used in electronic devices.
• In transformers and electric motors it is represented by a winding, which is made of this material.

There is no doubt that the expansion of the scope of application of this material will occur with further development technical progress. Although there are other materials besides copper, designers still use copper when creating equipment and various installations. main reason the demand for this material lies in good electrical and thermal conductivity this metal, which it provides at room temperature.

Temperature coefficient of resistance

All metals with any thermal conductivity have the property of decreasing conductivity with increasing temperature. As the temperature decreases, conductivity increases. Experts call the property of decreasing resistance with decreasing temperature particularly interesting. Indeed, in this case, when the temperature in the room drops to a certain value, the conductor may lose electrical resistance and it will move into the class of superconductors.

In order to determine the resistance value of a particular conductor of a certain weight at room temperature, there is a critical resistance coefficient. It is a value that shows the change in resistance of a section of a circuit when the temperature changes by one Kelvin. To calculate the electrical resistance of a copper conductor in a certain time period, use the following formula:

ΔR = α*R*ΔT, where α is the temperature coefficient of electrical resistance.

Conclusion

Copper is a material that is widely used in electronics. It is used not only in windings and circuits, but also as a metal for the manufacture of cable products. For machinery and equipment to work effectively, it is necessary correctly calculate the resistivity of the wiring, laid in the apartment. There is a certain formula for this. Knowing it, you can make a calculation that allows you to find out the optimal size of the cable cross-section. In this case, it is possible to avoid loss of equipment power and ensure its efficient use.

The main characteristics of conductor materials are:

1. Thermal conductivity;
2. Contact potential difference and thermoelectromotive force;
3. Tensile strength and tensile elongation.

ρ is a value characterizing the ability of a material to resist electric current. Specific resistance is expressed by the formula:

For long conductors (wires, cords, cable cores, busbars), the length of the conductor l usually expressed in meters, cross-sectional area S- in mm², conductor resistance r- in Ohm, then the dimension of resistivity

Data on the resistivities of various metal conductors are given in the article "Electrical resistance and conductivity".

α is a value characterizing the change in conductor resistance depending on temperature.
average value temperature coefficient of resistance in the temperature range t 2° - t 1° can be found by the formula:

The temperature coefficients of resistance of various conductor materials are given in the table below.

The value of temperature coefficients of resistance of metals

Thermal conductivity

λ is a quantity characterizing the amount of heat passing per unit time through a layer of matter. Thermal conductivity dimension

Thermal conductivity has great importance for thermal calculations of machines, apparatus, cables and other electrical devices.

Thermal conductivity value λ for some materials

Silver Copper Aluminum Brass Iron, steel Bronze Concrete Brick Glass Asbestos Tree Cork

350 - 360 340 180 - 200 90 - 100 40 - 50 30 - 40 0,7 - 1,2 0,5 - 1,2 0,6 - 0,9 0,13 - 0,18 0,1 - 0,15 0,04 - 0,08

From the data presented it is clear that metals have the greatest thermal conductivity. Non-metallic materials have significantly lower thermal conductivity. It reaches especially low values ​​for porous materials, which I use specifically for thermal insulation. According to the electronic theory, the high thermal conductivity of metals is due to the same conduction electrons as electrical conductivity.

Contact potential difference and thermoelectromotive force

As stated in the article “Metal Conductors”, positive metal ions are located at the nodes of the crystal lattice, forming, as it were, its frame. Free electrons fill the lattice like a gas, sometimes called "electron gas." The pressure of the electron gas in a metal is proportional to the absolute temperature and the number of free electrons per unit volume, which depends on the properties of the metal. When two dissimilar metals come into contact at the point of contact, the pressure of the electron gas equalizes. As a result of electron diffusion, a metal whose number of electrons decreases is charged positively, and a metal whose number of electrons increases is charged negatively. A potential difference occurs at the point of contact. This difference is proportional to the temperature difference between the metals and depends on their type. A thermoelectric current arises in a closed circuit. The electromotive force (EMF) that creates this current is called thermoelectromotive force(thermo-EMF).

The phenomenon of contact potential difference is used in technology to measure temperature using thermocouples. When measuring small currents and voltages in a circuit at the junction of different metals, a large potential difference may arise, which will distort the measurement results. In this case, it is necessary to select materials so that the measurement accuracy is high.

Tensile strength and tensile elongation

When choosing wires, in addition to the cross-section, wire material, and insulation, it is necessary to take into account their mechanical strength. This is especially true for overhead power lines. The wires are stretched. Under the influence of force applied to the material, the latter elongates. If we designate the original length l 1 and the final length l 2, then the difference l 1 - l 2 = Δ l will absolute elongation.

Attitude

called relative elongation.

The force that produces material rupture is called breaking load, and the ratio of this load to the cross-sectional area of ​​the material at the moment of destruction is called temporary tensile strength and is designated

Temporary tensile strength data for various materials are given below.

Tensile strength value for various metals

When heated, it increases as a result of an increase in the speed of movement of atoms in the conductor material with increasing temperature. The specific resistance of electrolytes and coal when heated, on the contrary, decreases, since in these materials, in addition to increasing the speed of movement of atoms and molecules, the number of free electrons and ions per unit volume increases.

Some alloys, which have more than their constituent metals, almost do not change their resistivity with heating (constantan, manganin, etc.). This is explained by the irregular structure of the alloys and the short mean free path of electrons.

The value showing the relative increase in resistance when the material is heated by 1° (or decreased when cooled by 1°) is called.

If the temperature coefficient is denoted by α, the resistivity at to = 20 o by ρ o, then when the material is heated to a temperature t1, its resistivity p1 = ρ o + αρ o (t1 - to) = ρ o(1 + (α (t1 -to))

and accordingly R1 = Ro (1 + (α (t1 - to))

Temperature coefficient a for copper, aluminum, tungsten is 0.004 1/deg. Therefore, when heated by 100°, their resistance increases by 40%. For iron α = 0.006 1/deg, for brass α = 0.002 1/deg, for fechral α = 0.0001 1/deg, for nichrome α = 0.0002 1/deg, for constantan α = 0.00001 1/deg , for manganin α = 0.00004 1/deg. Coal and electrolytes have a negative temperature coefficient of resistance. The temperature coefficient for most electrolytes is approximately 0.02 1/deg.

The property of conductors to change their resistance depending on temperature is used in resistance thermometers. By measuring the resistance, the ambient temperature is determined by calculation. Constantan, manganin and other alloys with a very small temperature coefficient of resistance are used for the manufacture of shunts and additional resistances to measuring instruments.

Example 1. How will the resistance Ro of an iron wire change when it is heated to 520°? Temperature coefficient a of iron is 0.006 1/deg. According to the formula R1 = Ro + Ro α (t1 - to) = Ro + Ro 0.006 (520 - 20) = 4Ro, that is, the resistance of the iron wire when heated by 520° will increase 4 times.

Example 2. Aluminum wires at a temperature of -20° have a resistance of 5 ohms. It is necessary to determine their resistance at a temperature of 30°.

R2 = R1 - α R1(t2 - t1) = 5 + 0.004 x 5 (30 - (-20)) = 6 ohms.

The property of materials to change their electrical resistance when heated or cooled is used to measure temperatures. So, thermal resistance, which are wires made of platinum or pure nickel, fused into quartz, are used to measure temperatures from -200 to +600°. Semiconductor thermal resistances with a large negative coefficient are used to accurately determine temperatures in narrower ranges.

Semiconductor thermal resistances used to measure temperatures are called thermistors.

Thermistors have a high negative temperature coefficient of resistance, that is, when heated, their resistance decreases. made from oxide (subject to oxidation) semiconductor materials consisting of a mixture of two or three metal oxides. The most common are copper-manganese and cobalt-manganese thermistors. The latter are more sensitive to temperature.