Any production involves the use of They are also necessary in everyday life: you must admit, it is difficult to do during repairs without the simplest measuring instruments, such as a ruler, tape measure, caliper, etc. Let's talk about what measuring tools and instruments exist, what are their fundamental differences and where certain types are used.

General information and terms

Measuring device - a device with which the value of a physical quantity is obtained in a given range, determined by the scale of the device. In addition, such a tool allows you to translate values, making them more understandable to the operator.

The control device is used to control the conduct of the technological process. For example, it can be some kind of sensor installed in a heating furnace, air conditioner, heating equipment, and so on. Such a tool often defines properties as well. Currently, a wide variety of devices are being produced, among which there are both simple and complex. Some have found their application in one, while others are used everywhere. To deal with this issue in more detail, it is necessary to classify this tool.

Analog and digital

Control and measuring devices and tools are divided into analog and digital. The second type is more popular, since various values, for example, current or voltage, are converted into numbers and displayed on the screen. This is very convenient and the only way to achieve high accuracy of readings. However, it must be understood that any digital instrument includes an analog converter. The latter is a sensor that takes readings and sends data to be converted into a digital code.

Analog measuring and control instruments are simpler and more reliable, but at the same time less accurate. Moreover, they are mechanical and electronic. The latter differ in that they incorporate amplifiers and transducers. They are preferred for a number of reasons.

Classification according to various criteria

Measuring tools and devices are usually divided into groups depending on the method of providing information. So, there are registering and showing instruments. The former are characterized by the fact that they are able to record readings in memory. Often, self-recording devices are used that independently print out data. The second group is intended exclusively for real-time monitoring, that is, while taking readings, the operator must be near the device. Also, the control and measuring tool is classified according to:

  • direct action - one or more quantities are converted without comparison with the same name;
  • comparative - a measuring tool designed to compare the measured value with the already known.

What are the devices in the form of presentation of indications (analog and digital), we have already figured out. Measuring instruments and devices are also classified according to other parameters. For example, there are summing and integrating, stationary and switchboard, standardized and non-standardized devices.

Measuring locksmith tools

We encounter such devices most often. The accuracy of the work is important here, and since a mechanical tool is used (for the most part), it is possible to achieve an error of 0.1 to 0.005 mm. Any unacceptable error leads to the need for regrinding or even replacement of a part or a whole assembly. That is why, when fitting the shaft to the bushing, the mechanic does not use rulers, but more precise tools.

The most popular locksmith measuring equipment is a caliper. But even such a relatively accurate device does not guarantee a 100% result. That is why experienced locksmiths always take a large number of measurements, after which it is selected. If you want to get more accurate readings, then use a micrometer. It allows measurements down to hundredths of a millimeter. However, many people think that this instrument is capable of measuring down to microns, which is not entirely true. And it is unlikely that such accuracy will be required when carrying out simple plumbing work at home.

About goniometers and probes

It is impossible not to talk about such a popular and effective tool as a goniometer. From the name you can understand that it is used if you want to accurately measure the corners of parts. The device consists of a half-disk with a marked scale. It has a ruler with a movable sector, on which the vernier scale is applied. To fix the movable sector of the ruler on the half-disk, a locking screw is used. The measurement process itself is quite simple. First you need to attach the measured part with one face to the ruler. In this case, the ruler is shifted so that a uniform gap is formed between the faces of the part and the rulers. After that, the sector is fixed with a locking screw. First of all, readings are taken from the main ruler, and then from the vernier.

Often a feeler gauge is used to measure the gap. It is an elementary set of plates fixed at one point. Each plate has its own thickness, which we know. By installing more or fewer plates, you can measure the gap quite accurately. In principle, all these measuring instruments are manual, but they are quite effective and it is hardly possible to replace them. And now let's go further.

A bit of history

It should be noted, considering measuring instruments: their types are very diverse. We have already studied the main devices, but now I would like to talk about a little about other tools. For example, an acetometer is used to measure strength. This device is able to determine the amount of free acetic acids in a solution, and was invented by Otto and was used throughout the 19th and 20th centuries. The acetometer itself is similar to a thermometer and consists of a 30x15cm glass tube. There is also a special scale, which allows you to determine the required parameter. However, today there are more advanced and accurate methods for determining the chemical composition of a liquid.

Barometers and ammeters

But almost every one of us is familiar with these tools from school, technical school or university. For example, a barometer is used to measure atmospheric pressure. Today, liquid and mechanical barometers are used. The first can be called professional, since their design is somewhat more complicated, and the readings are more accurate. Mercury barometers are used at weather stations because they are the most accurate and reliable. Mechanical options are good for their simplicity and reliability, but they are gradually being replaced by digital devices.

Instruments and instruments for measurements, such as ammeters, are also familiar to everyone. They are needed to measure the current strength in amperes. The scale of modern instruments is graduated in different ways: microamps, kiloamps, milliamps, etc. Ammeters always try to connect in series: this is necessary to lower the resistance, which will increase the accuracy of the readings taken.

Conclusion

So we talked with you about what control and measuring tools are. As you can see, all are different from each other and have a completely different scope. Some are used in meteorology, others in mechanical engineering, and still others in the chemical industry. Nevertheless, they have one goal - to measure the readings, record them and control the quality. To do this, it is advisable to use accurate measuring instruments. But this parameter also contributes to the fact that the device becomes more complex, and the measurement process depends on more factors.


Lab #2-0

ELECTRICAL INSTRUMENTS AND MEASUREMENT METHODS

The purpose of the work: to get acquainted with electrical measuring instruments and measurement methods.

ANSWERS TO CONTROL QUESTIONS

Electrical measuring instruments. Instrument types.

Electrical measuring instruments are used to control the operating mode of electrical installations, test them and account for the consumed electrical energy.

Depending on the purpose, electrical measuring instruments are divided into:

1) ammeters (current meters);

2) voltmeters (voltage meters);

3) wattmeters (power meters);

4) ohmmeters (resistance meters);

5) frequency meters (AC frequency meters);

6) electricity meters, etc.

Instrument types. Depending on the method of counting, devices are divided into devices direct reading (direct evaluation) and comparison devices. Direct reading devices are those that allow reading the measured value directly on the scale. These include ammeters, voltmeters, wattmeters, etc. The main part of each such device is a measuring mechanism. When a measured electrical quantity (current, voltage, power, etc.) acts on the measuring mechanism of the device, the arrow mounted on its axis rotates through a certain angle, according to which the value of the measured quantity is determined on the scale of the device.

In electrical comparison instruments, measurements are carried out by comparing the measured value with some exemplary measure (standard). These include bridges for measuring resistance and compensation measuring devices (potentiometers).



The action of electrical measuring instruments for direct evaluation is based on various manifestations of electric current (magnetic, thermal, electrodynamic, etc.). Let us note some features of the design of instrument parts. The scale is used to read the measured value. The numbers next to the divisions indicate either the number of divisions from zero on the scale (usually in instruments of 0.2; 0.5 accuracy classes), or directly the value of the measured value (other accuracy classes). In the first case, to obtain the value of the measured quantity in practical units, it is necessary to determine the price of one division of the instrument scale (sometimes called the instrument constant) and multiply it by the number of divisions counted. For example, we have a device that can measure voltage from 0 to 250 IN(Fig. 1).

Value of division: C = 250 / 50 = 5In / div.

Rice. 1.

Voltmeter scale from 0 to 250 V

When reading, the line of sight must be perpendicular to the scale, otherwise an error from parallax is possible. When reading on a mirror scale, the observer's eye should be positioned so that the end of the arrow covers its image in the mirror. In order to reduce the time required to calm the moving part of the device (after switching on), there are special braking devices (dampers).

Depending on the principle of operation, electrical measuring instruments are classified as magnetoelectric, electromagnetic, electrodynamic, thermoelectric, rectifier, induction and electrostatic systems. Each of these systems has a symbol.

LECTURE # 1

Subject:ELECTRICAL INSTRUMENTS AND ELECTRICAL MEASUREMENTS

1. General information about electrical measuring instruments

Electrical measuring instruments are designed to measure various quantities and parameters of an electrical circuit: voltage, current, power, frequency, resistance, inductance, capacitance and others.

In the diagrams, electrical measuring instruments are depicted by conventional graphic symbols in accordance with GOST 2.729-68. Figure 1.1 shows the general designations of indicating and recording instruments.

Rice. 1.1 Symbols for electrical measuring instruments.

To indicate the purpose of the electrical measuring instrument, a specifying symbol established in the standards or the letter designation of the units of measurement of the instrument according to GOST in accordance with Table 1.1 is entered into its general designation.

Table 1.1

Name

units

Symbol

Name

units

Symbol

Milliamp

microamp

Millivolt

Kilowatt

Power factor

2. Electromechanical measuring instruments

According to the principle of operation, electromechanical devices are divided into devices of magnetoelectric, electromagnetic, ferrodynamic, induction, electrostatic systems. Symbols of systems are given in table. 1.2. The most widely used devices are the first three types: magnetoelectric, electromagnetic, electrodynamic.

Table 1.2

Instrument type

Symbol

Type of measured current

Advantages

Flaws

electric

Constant

High precision, scale uniformity

Not resistant to overload

magnetic

Variable

constant

Simplicity of the device, resistant to overloads

Low accuracy, sensitive to noise

dynamic

Variable

constant

High accuracy

low sensitivity,

sensitive to interference

Induction

Variable

High reliability, overload resistant

Low accuracy

3. Fields of application of electromechanical devices

Magnetoelectric devices: panel and laboratory ammeters and voltmeters; zero indicators for measurements in bridge and compensation circuits.

In industrial low frequency AC installations, most ammeters and voltmeters are electromagnetic system instruments. Laboratory instruments of class 0.5 and more can be manufactured to measure direct and alternating currents and voltages.

Electrodynamic mechanisms are used in laboratory and exemplary instruments for measuring direct and alternating currents, voltages and powers.

Induction devices based on induction mechanisms are mainly used as single- and three-phase AC energy meters. By accuracy, the counters are divided into classes 1.0; 2.0; 2.5. The CO meter (single-phase meter) is used to account for active energy (watt-hours) in single-phase circuits. To measure active energy in three-phase circuits, two-element inductive meters are used, the counting mechanism of which takes into account kilowatt-hours. To account for reactive energy, special inductive meters are used, which have some changes in the winding arrangement or in the switching circuit.

Active and reactive meters are installed at all enterprises for settlement with energy supply organizations for the electricity used.

The principle of choosing measuring instruments

1. Determine the maximum values ​​of current, voltage and power in the circuit by calculating the circuit. Often the values ​​of the measured quantities are known in advance, for example, the mains or battery voltage.

2. Depending on the type of measured value, direct or alternating current, select the device system. For technical measurements of direct and alternating current, the magnetoelectric and electromagnetic systems are selected, respectively. In laboratory and precise measurements, a magnetoelectric system is used to determine direct currents and voltages, and an electrodynamic system is used to determine alternating current and voltage.

3. Select the measurement limit of the device so that
the measured value was in the last, third part of the scale
device.

4. Depending on the required measurement accuracy, choose a class
instrument accuracy.

4. Ways to include devices in the circuit

Ammeters are connected in series with the load in the circuit, voltmeters - in parallel, wattmeters and counters, as having two windings (current and voltage), are connected in series - in parallel (Fig. 1.2.).

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Rice. 1.3. Ways to expand the limits of measurement of devices.

The division price of multi-limit ammeters, voltmeters, wattmeters is determined by the formula:

P" in the most significant digit) and change the polarity of the input signal when the "-" sign flashes in the most significant digit.

Measurement error of the multimeter VR-11 A.

DC voltage: ±(0.5% Ux +4 digits).

AC voltage: ±(0.5% Ux + 10 digits),

where Ux - instrument reading;

zn. - low order unit.

Advantages of electronic devices: high input impedance, which allows measurements without affecting the circuit; wide measurement range, high sensitivity, wide frequency range, high measurement accuracy.

6. Errors of measurements and measuring instruments

It is customary to characterize the quality of means and measurement results by indicating their errors. There are about 30 types of errors. They are defined in the measurement literature. It should be borne in mind that the errors of measuring instruments and the errors of measurement results are not identical concepts. Historically, some of the names of the variety of errors have been assigned to the errors of measuring instruments, the other to the errors of measurement results, and some are applied to both.

The ways of presenting the error are as follows.

Depending on the tasks to be solved, several methods of representing the error are used, most often absolute, relative and reduced are used.

Absolute error – measured in the same units as the measured value. Characterizes the magnitude of the possible deviation of the true value of the measured value from the measured value.

Relative error is the ratio of the absolute error to the value of the quantity. If we want to determine the error over the entire measurement interval, we must find the maximum value of the ratio over the interval. It is measured in dimensionless units.

Accuracy class is the relative error, expressed as a percentage. Typically, the accuracy class values ​​are selected from the range: 0.1; 0.5: 1.0; 1.5; 2.0; 2.5 etc.

The concepts of absolute and relative errors apply to both measurements and measuring instruments, and the reduced error evaluates only the accuracy of measuring instruments.

The absolute measurement error is the difference between the measured x value and its true chi value:

Usually, the true value of the quantity being measured is unknown, and instead of it in (1.1) the value of the quantity measured by a more accurate instrument, i.e., having a smaller error than the instrument giving the value of x, is substituted. The absolute error is expressed in units of the measured value. Formula (1.1) is used when checking measuring instruments.

Relative error https://pandia.ru/text/78/613/images/image020_7.gif" width="99" height="45"> (1.2)

According to the relative measurement error, the measurement accuracy is assessed.

The reduced error of the measuring device is defined as the ratio of the absolute error to the normalizing value xn and is expressed as a percentage:

(1.3)

The normalizing value is usually taken equal to the upper limit of the working part of the scale, in which the zero mark is on the edge of the scale.

The given error determines the accuracy of the measuring device, does not depend on the measured value and has the only value for this device. Of (1..gif" width="15" height="19 src=">, the smaller the measured value x in relation to the measurement limit of the device xN, the more.

Many measuring instruments differ in accuracy classes. Instrument accuracy class G is a generalized characteristic that characterizes the accuracy of the instrument, but is not a direct characteristic of the measurement accuracy performed using this instrument.

The accuracy class of the device is numerically equal to the largest allowable reduced basic error, calculated as a percentage. For ammeters and voltmeters, the following accuracy classes are established: 0.05; 0.1; 0.2; 0.5; 1.0; 1.5; 2.5; 4.0; 5.0. These numbers are plotted on the scale of the instrument. For example, class 1 characterizes the guaranteed limits of error in percent (± 1%, for example, from the final value of 100 V, i.e. ± 1 V) under normal operating conditions.

According to the international classification, devices with an accuracy class of 0.5 or more are considered accurate or exemplary, and devices with an accuracy class of 1.0 or less are considered working. All devices are subject to periodic verification for compliance of metrological characteristics, including accuracy class, with their passport values. In this case, the exemplary instrument must be more accurate than the one verified through the class, namely: verification of an instrument with an accuracy class of 4.0 is carried out by an instrument with an accuracy class of 1.5, and verification of an instrument with an accuracy class of 1.0 is carried out by an instrument with an accuracy class of 0.2.

Since both the accuracy class of the device G and the measurement limit XN are given on the scale of the device, the absolute error of the device is determined from formula (1.3):

https://pandia.ru/text/78/613/images/image019_7.gif" width="15 height=19" height="19"> With instrument accuracy class G is expressed by the formula:

whence it follows that the relative measurement error is equal to the accuracy class of the device only when measuring the limiting value on the scale, i.e. when x = XN. With a decrease in the measured value, the relative error increases. How many times XN\u003e x, so many times\u003e G. Therefore, it is recommended to choose the measurement limits of the indicating instrument so as to read the readings within the last third of the scale, closer to its end.

7. Presentation of the measurement result for single measurements

The measurement result consists of an estimate of the measured quantity and a measurement error characterizing the accuracy of the measurement. According to GOST 8.011-72, the measurement result is presented in the form:

where A is the measurement result;

Absolute error of the device;

P - probability, in case of statistical data processing.

At the same time, A and https://pandia.ru/text/78/613/images/image023_5.gif" width="15" height="17"> should not have more than two significant digits.

Electronic measuring instruments have increased speed, high sensitivity and a fairly wide frequency range. They are used to measure certain electrical quantities - voltage, current, resistance and other parameters.

These devices are divided into analog and digital models. These models differ from each other in that they have a different form of information reproduction - using a digital monitor or an arrow. To date, electronic digital measuring instruments are the most popular, since mechanical options lose in the correctness of the displayed information. However, the affordable cost of many inclines towards the purchase of mechanical devices.

Voltage gauges and indicators

Used to determine the presence or absence of current in the network for electrical appliances, the power of which is not more than 1000 V. The principle of operation is the conversion of electrical signals into light signals. The device has a scale and a light indicator, with which you can simply understand whether there is voltage in the network. If there is no glow, then this indicates its breakage or absence. The indicators can also measure the phases of the AC current and the polarity of the DC current.

Voltmeter, ammeter, ohmmeter

An electronic device is used to measure current strength, voltage, power, resistance, capacitance, inductance, etc. They can combine converters from the measured value to DC voltage, that is, current strength, they can also combine a magnetoelectric device and differ in high sensitivity, wide frequency range and low power consumption.

Through the divider, the determined voltage is supplied to the output of the amplifier, and the output voltage after the amplifier is calculated by the magnetoelectric apparatus. The main error of this voltmeter is 0.5 ... 1.0 percent.

An AC voltmeter is an electronic device designed to measure and convert AC voltage to DC voltage. Voltmeters are divided depending on the measured alternating voltage: RMS values, average rectifier values ​​and amplitude values.

The ohmmeter is not available as a separate device; its functions are performed by an electronic voltmeter. The ohmmeter is equipped with a converter, which is an amplifier surrounded by a negative feedback of the measured and exemplary resistors. Therefore, the voltage measured by an electronic voltmeter is proportional to the resistance of the resistor being determined. Such a circuit is very popular for measuring resistance from 10 to 1000 MΩ.

Frequency meter and oscilloscope

The frequency meter adopts the principle of charge and discharge of a capacitor and is combined with an analog output mechanism designed to determine the average amount of force flowing through the capacitor during its periodic recharging relative to the detected frequency.

In order to investigate the behavior of signals over time, an electronic oscilloscope is used, which makes it possible to directly observe or record the shape of non-periodic and periodic signals. Due to the fact that in the oscilloscope the moving part is an electronic meadow, it has practically no inertia and can be used to measure quantities with a frequency of up to several hundred megahertz and non-periodic operations, the duration of which reaches a fraction of microseconds.

These devices for measuring current and voltage also have a large input resistance and high sensitivity. However, they also have disadvantages, namely, low measurement accuracy (10 percent error), structural and electrical complexity, and high cost. Moreover, if we compare the oscilloscope with other electronic measuring instruments, then it is the most difficult to operate and requires a certain qualification of personnel.

The oscilloscope has become widespread due to the measurements of the phase and frequency of electrical oscillations. In addition, it is possible to explore vibrations of various shapes.

As a rule, this device is used for a short current measurement without breaking the circuit. Due to the fact that current is supplied to the coil from the line being determined, it is possible not to break the circuit during operation - this is the primary principle of operation of this electronic device. Clamp meters can be analog or digital. The main functions that they perform: measuring AC voltage, DC voltage, resistance, AC current, temperature.

This is a device that combines almost all devices designed to measure current and voltage, as well as other parameters. It can contain an ammeter, and a voltmeter, and an ohmmeter, and similar electronic devices. Due to their simple design and positive properties, these multimeters have been very well known for many years. Multimeters come in varying degrees of accuracy, which directly affects their cost, so before choosing this electrical measuring instrument, you need to decide on the tasks that it will perform.

Repair of electronic devices

Due to the fact that the designs of measuring instruments are diverse, it is very difficult to describe all the disassembly and assembly processes. However, most processes are common to any instrument design.

Homogeneous repair processes can be performed by specialists of different qualifications. Devices of class 1 - 1.5 - 2.5 - 4 must be repaired by masters whose qualification is 4-6 category. Complex and special devices must be repaired by electromechanics of the 7th-8th category.

In general, the processes of disassembling and assembling electrical measuring instruments are responsible processes, so they must be carried out carefully and carefully. In the case of careless disassembly, individual parts may deteriorate, which will lead to the addition of new faults. Before starting disassembly, you should consider the general procedure for carrying out operations.

A complete disassembly of an electronic device is performed during a major overhaul, which is associated with the rewinding of coils, frames, resistances, the production or replacement of destroyed and burnt parts. It provides for the separation of all parts of the device among themselves.

When an average repair is carried out, an incomplete disassembly of all parts of the device is carried out, and they are limited only to removing the moving part, changing the thrust bearings, refueling the cores, restoring the moving part, adjusting and adjusting the readings of the mechanism. Recalibration during a medium repair should only be done when the scale is dull and dirty. In other cases, the scale should be kept with the same marks. An indicator of a high-quality average repair is the production of a device with the same scale.

To perform disassembly and assembly of devices, you will need watch tweezers, screwdrivers, small electric soldering irons, watch cutters, oval nose pliers, pliers, specially made keys, etc.

After a complete repair of the device, it is checked whether the moving part moves freely, the inside is inspected, and the readings of the repaired and exemplary apparatus are recorded during measurements of the determined value from zero to maximum and back.

Measurement is the determination of the value of a physical quantity empirically using special technical means. Measurements are made in generally accepted units.

The main elements of the measurement process: measurement object, measured value, measuring instrument, measurement principle, measurement method, measurement conditions, measurement result, measurement error, human operator performing measurements (subject of measurement).

Measurement object- this is a complex, multifaceted phenomenon or process (for example, electrical oscillations at the output of an autogenerator), characterized by many individual physical parameters. One of these parameters that interests us and is to be measured is called the measured physical quantity (for example, the oscillation frequency of an oscillator).

measuring instrument- this is a technical tool used in measurements and having normalized metrological properties.

Measuring principle is a set of physical phenomena on which measurements are based (for example, the resonant principle of frequency measurement).

Measurement method is a set of techniques for using the principles and means of measurement (for example, the method of comparing the measured frequency with a known frequency).

Electrical methods for measuring electrical and non-electrical quantities have a number of advantages over other measurement methods: low energy consumption; the possibility of remote transmission of measurement information; high measurement speed; high precision and sensitivity.

Measurement technique unlike the method, it includes a detailed procedure for the measurement process using specific methods and measuring instruments.

No matter how carefully the measurement is carried out, its result will contain some inaccuracy, which is characterized by an error. Measurement error is the deviation of the measurement result from the true value of the measured quantity.

The widely used term measurement accuracy characterizes the quality of measurements, reflecting the closeness of their results to the true value. Greater accuracy corresponds to a smaller measurement error.

The value of a physical quantity found by measuring it is called the measurement result. The measurement result can be obtained as a result of one observation or by processing the results of several

observations. In this case, observation is understood as an experimental operation in which one numerical value of a quantity is obtained.

The Republic of Belarus has introduced the International System of Units, abbreviated SI (SI). The basic units of this system are: meter ( m), kilogram ( kg), second ( s), ampere ( A), kel-vin ( TO), mol ( mol) and candela ( cd), additional - angular units: radian (rad) and steradian (sr). In addition to the main and additional units, derivative units are established.


Technical means used in electrical measurements and having normalized errors are divided by purpose into measures, measuring transducers, electrical measuring instruments, electrical measuring installations and measuring systems.

A measure is a means of measurement designed to reproduce the value of a physical quantity of a given size with a certain accuracy. There are unambiguous measures, for example, a resistance measuring coil, a capacitor, and multi-valued (variable value), as well as sets and stores of measures, i.e., sets of measures for reproducing a number of the same values ​​of quantities of various sizes (resistance stores, capacitances).

Measuring transducers are designed to generate measurement information signals in a form convenient for transmission, further transformation and processing, but not amenable to direct perception by the observer. Some of them - shunts, voltage dividers, instrument transformers, amplifiers - can convert electrical quantities into electrical ones, but necessary for the consumer, others - thermoelectric thermometers, strain gauges, inductive converters - non-electric quantities into electrical ones.

Electrical measuring instruments are electrical measuring instruments designed to generate measurement information signals in a form convenient for direct perception by the observer (for example, a voltmeter, ammeter, wattmeter, phase meter).

Electrical measuring instruments are classified according to their purpose, design, type of measured value, principle, operating conditions, accuracy class and other criteria.

Depending on the type of measured quantity (for example, voltage, current, power), electrical measuring instruments are divided into ammeters, voltmeters, wattmeters, etc., and combined, measuring two or more quantities (for example, ampere-voltmeters).

Electrical measuring instruments, the readings of which are continuous functions of the measured values, are called analog instruments. Electrical measuring instruments that automatically generate discrete signals of measuring information, the readings of which are presented in digital form, are called digital instruments.

An electrical measuring installation consists of a number of measuring instruments (measures, measuring transducers, devices) and auxiliary devices located in one place. Electrical measuring installations are used for verification and calibration of electrical measuring instruments and testing of magnetic and electrical insulating materials.

Depending on the method of obtaining the result, two methods of measurement are distinguished: straight And indirect.

Direct a measurement is called, the result of which is obtained directly from experimental data. This includes measurements of various physical quantities using instruments graduated in established units, for example, measuring current with an ammeter, conductor resistance with an ohmmeter, temperature with a thermometer, etc. Direct measurements are widely used because of their simplicity and speed of obtaining results.

Indirect a measurement is called, in which the desired value of a quantity is determined on the basis of a known mathematical relationship between it and the quantities obtained by direct measurements. For example, the power P in DC circuits is calculated by the formula: R= U I; voltage U in this case, measure with a voltmeter, and the current I- ammeter; resistor resistance R = U/I– by measured voltage values U and current I. Indirect measurements are used, as a rule, only in cases where direct measurements cannot be used.