Resistivity of aluminum at a temperature of 35 degrees. Copper resistance depending on temperature

Many people have heard about Ohm's law, but not everyone knows what it is. The study begins with school course physics. They are taught in more detail at the Faculty of Physics and Electrodynamics. This knowledge is unlikely to be useful to the average person, but it is necessary for general development, and for someone future profession. On the other hand, basic knowledge about electricity, its structure, and its features at home will help protect yourself from harm. It is not for nothing that Ohm’s law is called the fundamental law of electricity. For the home handyman You need to have knowledge in the field of electricity to prevent overvoltage, which can lead to an increase in load and a fire.

Concept of electrical resistance

The relationship between the basic physical quantities of an electrical circuit - resistance, voltage, current strength - was discovered by the German physicist Georg Simon Ohm.

The electrical resistance of a conductor is a value that characterizes its resistance to electric current. In other words, some of the electrons under the influence of electric current on the conductor leave their place in the crystal lattice and are directed to the positive pole of the conductor. Some electrons remain in the lattice, continuing to rotate around the nuclear atom. These electrons and atoms form electrical resistance that prevents the movement of released particles.

The above process applies to all metals, but resistance occurs differently in them. This is due to the difference in size, shape, and material of which the conductor is made. Accordingly, the dimensions of the crystal lattice have different shapes for different materials, therefore, the electrical resistance to the movement of current through them is not the same.

From this concept it follows that the specific resistance of a substance is determined, which is individual indicator for each metal separately. Electrical resistivity (SER) is a physical quantity, denoted by the Greek letter ρ, and characterized by the ability of a metal to prevent the passage of electricity through it.

Copper is the main material for conductors

The resistivity of a substance is calculated using the formula, where one of the important indicators is the temperature coefficient of electrical resistance. The table contains the resistivity values ​​of three known metals in the temperature range from 0 to 100°C.

If we take the resistivity indicator of iron as one of available materials, equal to 0.1 Ohm, then for 1 Ohm you will need 10 meters. Silver has the lowest electrical resistance; for its value of 1 ohm it will be 66.7 meters. A significant difference, but silver is an expensive metal that is not practical to use everywhere. The next best indicator is copper, where 57.14 meters are required per 1 ohm. Due to its availability and cost compared to silver, copper is one of the popular materials for use in electrical networks. The low resistivity of copper wire or the resistance of copper wire makes it possible to use copper conductor in many branches of science, technology, as well as for industrial and domestic purposes.

Resistivity value

The resistivity value is not constant; it varies depending on the following factors:

• Size. The larger the diameter of the conductor, the more electrons it allows through itself. Therefore, the smaller its size, the greater the resistivity.
• Length. Electrons pass through atoms, so the longer the wire, the more electrons have to travel through them. When calculating, it is necessary to take into account the length and size of the wire, because the longer, thinner wire, the greater its resistivity and vice versa. Failure to calculate the load of the equipment used can lead to overheating of the wire and a fire.
• Temperature. It is known that temperature regime has a big impact on the behavior of substances in different ways. Metal, like nothing else, changes its properties at different temperatures. The resistivity of copper directly depends on the temperature coefficient of resistance of copper and increases when heated.
• Corrosion. The formation of corrosion significantly increases the load. This happens due to the impact environment, ingress of moisture, salt, dirt, etc. manifestations. It is recommended to insulate and protect all connections, terminals, twists, install protection for equipment located on the street, and promptly replace damaged wires, components, and assemblies.

Resistance calculation

Calculations are made when designing objects for various purposes and uses, because everyone’s life support is provided by electricity. Everything is taken into account, from lighting fixtures to technically complex equipment. At home, it would also be useful to make a calculation, especially if it is planned to replace the electrical wiring. For private housing construction, it is necessary to calculate the load, otherwise the “makeshift” assembly of electrical wiring can lead to a fire.

The purpose of the calculation is to determine the total resistance of the conductors of all devices used, taking into account their technical parameters. It is calculated using the formula R=p*l/S, where:

R – calculated result;

p – resistivity indicator from the table;

l – length of wire (conductor);

S – section diameter.

Units

IN international system units of physical quantities (SI), electrical resistance is measured in Ohms (Ohm). The unit of measurement of resistivity according to the SI system is equal to the resistivity of a substance at which a conductor made of one material 1 m long with a cross-section of 1 sq. m. has a resistance of 1 Ohm. The use of 1 ohm/m for different metals is clearly shown in the table.

Significance of resistivity

The relationship between resistivity and conductivity can be considered as reciprocal quantities. The higher the indicator of one conductor, the lower the indicator of the other and vice versa. Therefore, when calculating electrical conductivity, the calculation 1/r is used, because the inverse of X is 1/X and vice versa. The specific indicator is denoted by the letter g.

Advantages of Electrolytic Copper

Copper is not limited to its low resistivity index (after silver) as an advantage. It has properties unique in its characteristics, namely plasticity and high malleability. Thanks to these qualities, electrolytic copper is produced to a high degree of purity for the production of cables that are used in electrical appliances, computer equipment, the electrical industry and the automotive industry.

Dependence of resistance index on temperature

Temperature coefficient is a quantity that is equal to the change in the voltage of a part of the circuit and the resistivity of the metal as a result of temperature changes. Most metals tend to increase resistivity with increasing temperature due to thermal vibrations of the crystal lattice. The temperature coefficient of resistance of copper affects the resistivity of copper wire and at temperatures from 0 to 100°C is 4.1 10− 3(1/Kelvin). At the silver this indicator under the same conditions it has a value of 3.8, and for iron it is 6.0. This once again proves the effectiveness of using copper as a conductor.

Electrical resistivity is physical quantity, which shows the extent to which a material can resist the passage of electric current through it. Some people may get confused this characteristic with ordinary electrical resistance. Despite the similarity of concepts, the difference between them is that specific refers to substances, and the second term refers exclusively to conductors and depends on the material of their manufacture.

Reciprocal of this material is the specific electrical conductivity. The higher this parameter, the better the current flows through the substance. Accordingly, the higher the resistance, the more losses are expected at the output.

Calculation formula and measurement value

Considering how specific electrical resistance is measured, it is also possible to trace the connection with non-specific, since units of Ohm m are used to denote the parameter. The quantity itself is denoted as ρ. With this value, it is possible to determine the resistance of a substance in a particular case, based on its size. This unit of measurement corresponds to the SI system, but other variations may occur. In technology you can periodically see the outdated designation Ohm mm 2 /m. To convert from this system to the international one, you will not need to use complex formulas, since 1 Ohm mm 2 /m equals 10 -6 Ohm m.

The formula for electrical resistivity is as follows:

R= (ρ l)/S, where:

• R – conductor resistance;
• Ρ – resistivity of the material;
• l – conductor length;
• S – conductor cross-section.

Temperature dependence

Electrical resistivity depends on temperature. But all groups of substances manifest themselves differently when it changes. This must be taken into account when calculating wires that will operate under certain conditions. For example, outdoors, where temperature values ​​depend on the time of year, necessary materials with less susceptibility to changes in the range from -30 to +30 degrees Celsius. If you plan to use it in equipment that will operate under the same conditions, then you also need to optimize the wiring for specific parameters. The material is always selected taking into account the use.

In the nominal table, electrical resistivity is taken at a temperature of 0 degrees Celsius. The increase in the indicators of this parameter when the material is heated is due to the fact that the intensity of the movement of atoms in the substance begins to increase. Electric charge carriers scatter randomly in all directions, which leads to the creation of obstacles to the movement of particles. The amount of electrical flow decreases.

As the temperature decreases, the conditions for current flow become better. Upon reaching a certain temperature, which will be different for each metal, superconductivity appears, at which the characteristic in question almost reaches zero.

The differences in parameters sometimes reach very large values. Those materials that have high performance can be used as insulators. They help protect wiring from short circuits and unintentional human contact. Some substances are not applicable at all for electrical engineering if they have a high value of this parameter. Other properties may interfere with this. For example, the electrical conductivity of water will not have of great importance for this area. Here are the values ​​of some substances with high indicators.

Substances with low performance are used more actively in electrical engineering. These are often metals that serve as conductors. There are also many differences between them. To find out the electrical resistivity of copper or other materials, it is worth looking at the reference table.

Specific volumetric electrical resistivity

This parameter characterizes the ability to pass current through the volume of a substance. To measure, it is necessary to apply a voltage potential from different sides of the material from which the product will be included in the electrical circuit. It is supplied with current with rated parameters. After passing, the output data is measured.

Use in electrical engineering

Changing a parameter at different temperatures is widely used in electrical engineering. Most simple example is an incandescent lamp that uses a nichrome filament. When heated, it begins to glow. When current passes through it, it begins to heat up. As heating increases, resistance also increases. Accordingly, the initial current that was needed to obtain lighting is limited. A nichrome spiral, using the same principle, can become a regulator on various devices.

Widespread use has also affected noble metals, which have suitable characteristics for electrical engineering. For critical circuits that require high speed, silver contacts are selected. They are expensive, but given the relatively small amount of materials, their use is quite justified. Copper is inferior to silver in conductivity, but has more affordable price, due to which it is more often used to create wires.

In conditions where maximum use can be made low temperatures, superconductors are used. For room temperature and outdoor use they are not always appropriate, since as the temperature rises their conductivity will begin to fall, so for such conditions aluminum, copper and silver remain the leaders.

In practice, many parameters are taken into account and this is one of the most important. All calculations are carried out at the design stage, for which reference materials are used.

When an electrical circuit is closed, at the terminals of which there is a potential difference, an electric current occurs. 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 electrons continuously changes: when electrons collide with atoms, molecules and other electrons, it decreases, then under the influence electric field increases and decreases again with a new collision. As a result, a uniform flow of electrons is established in the conductor 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 electric current passes through a conductor, the latter heats up.

Electrical resistance

The electrical resistance of a conductor, which is designated Latin letter r, is the property of a body or medium to convert electrical energy into thermal energy 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 various 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 denoted by the 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– conductor length 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 is 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 more resistance a conductor has, the less conductivity it has, the worse it conducts electric current, and, conversely, the lower the resistance of a conductor, the more 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 symbolized 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)

Resistivity is an applied concept in electrical engineering. It denotes how much resistance per unit length a material of a unit cross-section has to the current flowing through it - in other words, what resistance a wire of a millimeter cross-section one meter long has. This concept is used in various electrical calculations.

It is important to understand the differences between DC electrical resistivity and AC electrical resistivity. In the first case, the resistance is caused solely by the action of direct current on the conductor. In the second case, alternating current (it can be of any shape: sinusoidal, rectangular, triangular or arbitrary) causes an additional vortex field in the conductor, which also creates resistance.

Physical representation

In technical calculations involving the laying of cables of various diameters, parameters are used to calculate the required cable length and its electrical characteristics. One of the main parameters is resistivity. Electrical resistivity formula:

ρ = R * S / l, where:

• ρ is the resistivity of the material;
• R is the ohmic electrical resistance of a particular conductor;
• S - cross section;
• l - length.

The dimension ρ is measured in Ohm mm 2 /m, or, to abbreviate the formula - Ohm m.

The value of ρ for the same substance is always the same. Therefore, this is a constant characterizing the material of the conductor. It is usually indicated in directories. Based on this, it is already possible to calculate technical quantities.

It is important to say about specific electrical conductivity. This value is the inverse of the resistivity of the material, and is used equally with it. It is also called electrical conductivity. The higher this value, the better the metal conducts current. For example, the conductivity of copper is 58.14 m/(Ohm mm2). Or, in SI units: 58,140,000 S/m. (Siemens per meter is the SI unit of electrical conductivity).

We can talk about resistivity only in the presence of elements that conduct current, since dielectrics have infinite or close to infinite electrical resistance. In contrast, metals are very good conductors of current. You can measure the electrical resistance of a metal conductor using a milliohmmeter, or an even more accurate microohmmeter. The value is measured between their probes applied to the conductor section. They allow you to check circuits, wiring, windings of motors and generators.

Metals vary in their ability to conduct current. The resistivity of various metals is a parameter that characterizes this difference. The data is given at a material temperature of 20 degrees Celsius:

The parameter ρ shows what resistance a meter conductor with a cross section of 1 mm 2 will have. The higher this value, the greater the electrical resistance of the desired wire of a certain length. The smallest ρ, as can be seen from the list, is silver; the resistance of one meter of this material will be equal to only 0.015 Ohms, but this is too expensive a metal to use on an industrial scale. Next comes copper, which is much more common in nature (not a precious metal, but a non-ferrous metal). Therefore, copper wiring is very common.

Copper is not only a good conductor of electric current, but also a very ductile material. Thanks to this property, copper wiring fits better and is resistant to bending and stretching.

Copper is in great demand on the market. Many different products are made from this material:

• A huge variety of conductors;
• Auto parts (eg radiators);
• Clock mechanisms;
• Computer components;
• Parts of electrical and electronic devices.

The electrical resistivity of copper is one of the best among current-conducting materials, so many electrical industry products are created based on it. In addition, copper is easy to solder, so it is very common in amateur radio.

The high thermal conductivity of copper allows it to be used in cooling and heating devices, and its plasticity makes it possible to create the smallest details and the finest conductors.

Conductors of electric current are of the first and second kind. Conductors of the first kind are metals. Conductors of the second type are conductive solutions of liquids. The current in the first type is carried by electrons, and the current carriers in conductors of the second type are ions, charged particles of the electrolytic liquid.

We can only talk about the conductivity of materials in the context of ambient temperature. At a higher temperature, conductors of the first type increase their electrical resistance, and the second, on the contrary, decrease. Accordingly, there is a temperature coefficient of resistance of materials. The resistivity of copper Ohm m increases with increasing heating. The temperature coefficient α also depends only on the material; this value has no dimension and for different metals and alloys is equal to the following indicators:

• Silver - 0.0035;
• Iron - 0.0066;
• Platinum - 0.0032;
• Copper - 0.0040;
• Tungsten - 0.0045;
• Mercury - 0.0090;
• Constantan - 0.000005;
• Nickelin - 0.0003;
• Nichrome - 0.00016.

Determination of the electrical resistance value of a conductor section at elevated temperature R (t) is calculated using the formula:

R (t) = R (0) · , where:

• R (0) - resistance at initial temperature;
• α - temperature coefficient;
• t - t (0) - temperature difference.

For example, knowing the electrical resistance of copper at 20 degrees Celsius, you can calculate what it will be equal to at 170 degrees, that is, when heated by 150 degrees. The initial resistance will increase by a factor of 1.6.

As the temperature increases, the conductivity of materials, on the contrary, decreases. Since this is the reciprocal of electrical resistance, it decreases by exactly the same amount. For example, the electrical conductivity of copper when the material is heated by 150 degrees will decrease by 1.6 times.

There are alloys that practically do not change their electrical resistance when temperature changes. This is, for example, constantan. When the temperature changes by one hundred degrees, its resistance increases by only 0.5%.

While the conductivity of materials deteriorates with heat, it improves with decreasing temperature. This is related to the phenomenon of superconductivity. If you lower the temperature of the conductor below -253 degrees Celsius, its electrical resistance will sharply decrease: almost to zero. In this regard, the costs of transmitting electrical energy are falling. The only problem was cooling the conductors to such temperatures. However, due to the recent discoveries of high-temperature superconductors based on copper oxides, materials have to be cooled to acceptable values.

Resistivity of metals is a measure of their ability to resist the passage of electric current. This value is expressed in Ohm-meter (Ohm⋅m). The symbol for resistivity is the Greek letter ρ (rho). High resistivity means the material is a poor conductor of electrical charge.

Resistivity

Electrical resistivity is defined as the ratio between the electric field strength inside a metal and the current density within it:

Where:
ρ—metal resistivity (Ohm⋅m),
E - electric field strength (V/m),
J is the value of electric current density in the metal (A/m2)

If the electric field strength (E) in a metal is very high and the current density (J) is very small, this means that the metal has high resistivity.

The reciprocal of resistivity is electrical conductivity, which indicates how well a material conducts electric current:

σ is the conductivity of the material, expressed in siemens per meter (S/m).

Electrical resistance

Electrical resistance, one of the components, is expressed in ohms (Ohm). It should be noted that electrical resistance and resistivity are not the same thing. Resistivity is a property of a material, while electrical resistance is a property of an object.

The electrical resistance of a resistor is determined by a combination of its shape and the resistivity of the material from which it is made.

For example, a wire resistor made from a long and thin wire has a higher resistance than a resistor made from a short and thick wire of the same metal.

At the same time, a wirewound resistor made of a high resistivity material has greater electrical resistance than a resistor made of a low resistivity material. And all this despite the fact that both resistors are made of wire of the same length and diameter.

To illustrate this, we can draw an analogy with a hydraulic system, where water is pumped through pipes.

• The longer and thinner the pipe, the greater the resistance to water.
• A pipe filled with sand will resist water more than a pipe without sand.

Wire resistance

The amount of wire resistance depends on three parameters: the resistivity of the metal, the length and diameter of the wire itself. Formula for calculating wire resistance:

Where:
R - wire resistance (Ohm)
ρ - metal resistivity (Ohm.m)
L - wire length (m)
A - cross-sectional area of ​​the wire (m2)

As an example, consider a nichrome wirewound resistor with a resistivity of 1.10×10-6 Ohm.m. The wire has a length of 1500 mm and a diameter of 0.5 mm. Based on these three parameters, we calculate the resistance of the nichrome wire:

R=1.1*10 -6 *(1.5/0.000000196) = 8.4 Ohm

Nichrome and constantan are often used as resistance materials. Below in the table you can see the resistivity of some of the most commonly used metals.

Surface resistance

The surface resistance value is calculated in the same way as the wire resistance. IN in this case The cross-sectional area can be represented as the product of w and t:

For some materials, such as thin films, the relationship between resistivity and film thickness is called sheet sheet resistance RS:

where RS is measured in ohms. For this calculation, the film thickness must be constant.

Often, resistor manufacturers cut tracks into the film to increase resistance to increase the path for electrical current.

Properties of resistive materials

The resistivity of a metal depends on temperature. Their values ​​are usually given for room temperature (20°C). The change in resistivity as a result of a change in temperature is characterized by a temperature coefficient.

For example, thermistors (thermistors) use this property to measure temperature. On the other hand, in precision electronics, this is a rather undesirable effect.
Metal film resistors have excellent temperature stability properties. This is achieved not only due to the low resistivity of the material, but also due to the mechanical design of the resistor itself.

Many different materials and alloys are used in the manufacture of resistors. Nichrome (an alloy of nickel and chromium), due to its high resistivity and resistance to oxidation at high temperatures, is often used as a material for making wirewound resistors. Its disadvantage is that it cannot be soldered. Constantan, another popular material, is easy to solder and has a lower temperature coefficient.