What does it mean to measure a physical quantity? What is the unit of physical quantity? Here you will find answers to these very important questions.
1. Find out what is called a physical quantity
Since ancient times, people have used their characteristics to more accurately describe any events, phenomena, properties of bodies and substances. For example, comparing the bodies that surround us, we say that a book is smaller than a bookcase, and a horse is larger than a cat. This means that the volume of the horse is greater than the volume of the cat, and the volume of the book is less than the volume of the cabinet.
Volume is an example of a physical quantity that characterizes the general property of bodies to occupy one or another part of space (Fig. 1.15, a). In this case, the numerical value of the volume of each of the bodies is individual.
Rice. 1.15 To characterize the properties of bodies to occupy one or another part of space, we use the physical quantity volume (o, b), to characterize the movement - speed (b, c)
A general characteristic of many material objects or phenomena, which can acquire an individual meaning for each of them, is called physical quantity.
Another example of a physical quantity is the well-known concept of “speed”. All moving bodies change their position in space over time, but the speed of this change is different for each body (Fig. 1.15, b, c). Thus, an airplane manages to change its position in space by 250 m in 1 from a flight, a car - by 25 m, a person - by 1 m, and a turtle - by only a few centimeters. Therefore, physicists say that speed is a physical quantity that characterizes the speed of movement.
It is easy to guess that volume and speed are far from all the physical quantities that physics operates with. Mass, density, force, temperature, pressure, voltage, illumination - this is only a small part of those physical quantities that you will get acquainted with while studying physics.
2. Find out what it means to measure a physical quantity
In order to quantitatively describe the properties of any material object or physical phenomenon, it is necessary to establish the value of the physical quantity that characterizes this object or phenomenon.
The value of physical quantities is obtained by measurements (Fig. 1.16-1.19) or calculations.
Rice. 1.16. “There are 5 minutes left before the train departs,” you measure the time with excitement
Rice. 1.17 “I bought a kilo of apples,” says mom about her weight measurements
Rice. 1.18. “Dress warmly, it’s cooler outside today,” your grandmother takes care of you after measuring the air temperature outside
Rice. 1.19. 'My blood pressure's back up,' woman complains after taking her blood pressure
To measure a physical quantity means to compare it with a homogeneous quantity taken as a unit. 
Rice. 1.20 If a grandmother and grandson measure the distance in steps, they will always get different results
Let us give an example from fiction: “After walking three hundred paces along the river bank, a small detachment entered under the vaults of a dense forest, along the winding paths of which they had to wander for ten days.” (J. Verne "Fifteen-year-old captain")
Rice. 1.21.
The heroes of the novel by J. Verne measured the distance traveled, comparing it with a step, that is, a step served as a unit of measurement. There were three hundred such steps. As a result of the measurement, a numerical value (three hundred) of a physical quantity (path) in selected units (steps) was obtained.
Obviously, the choice of such a unit does not allow comparing the results of measurements obtained by different people, since the step length is different for everyone (Fig. 1.20). Therefore, for the sake of convenience and accuracy, people long ago began to agree on how to measure the same physical quantity with the same units. Today, in most countries of the world, the International System of Units of Measurement adopted in 1960, which is called the “International System” (SI) (Fig. 1.21), is in force.
In this system, the unit of length is the meter (m), time is the second (s); volume is measured in cubic meters (m 3), and speed is measured in meters per second (m / s). You will learn about other SI units later.
3. Remember multiples and submultiples
From the course of mathematics, you know that to reduce the notation of large and small values of different quantities, multiples and submultiples are used.
Multiple units are units that are 10, 100, 1000 or more times larger than base units. Submultiple units are units that are 10, 100, 1000 or more times smaller than the main ones.
Prefixes are used to record multiples and submultiples. For example, units of length, multiples of one meter, are a kilometer (1000 m), a decameter (10 m).
Units of length, sub-multiples of one meter, are decimeter (0.1 m), centimeter (0.01 m), micrometer (0.000001 m) and so on.
The table shows the most commonly used prefixes.
4. Getting to know measuring instruments
Scientists measure physical quantities using measuring instruments. The simplest of them - a ruler, tape measure - are used to measure the distance and linear dimensions of the body. You are also well aware of such measuring instruments as a clock - a device for measuring time, a protractor - a device for measuring angles on a plane, a thermometer - a device for measuring temperature, and some others (Fig. 1.22, p. 20). You have yet to become familiar with many measuring instruments.
Most measuring instruments have a scale that makes it possible to measure. In addition to the scale, the instrument indicates the units in which the value measured by this instrument is expressed *.
On the scale, you can set the two most important characteristics of the device: the limits of measurement and the division value.
Measurement limits- this is the largest and smallest values of a physical quantity that can be measured by this device.
Today, electronic measuring instruments are widely used, in which the value of the measured quantities is displayed on the screen in the form of numbers. Limits of measurement and units are determined by the passport of the device or set by a special switch on the panel of the device.
Rice. 1.22. Measuring instruments
Value of division- this is the value of the smallest division of the scale of the measuring instrument.
For example, the upper limit of measurements of a medical thermometer (Fig. 1.23) is 42 ° C, the lower one is 34 ° C, and the division value of the scale of this thermometer is 0.1 ° C.
We remind you: in order to determine the price of a division of the scale of any device, it is necessary to divide the difference between any two values \u200b\u200bof the quantities indicated on the scale by the number of divisions between them.
Rice. 1.23. Medical thermometer
- Summing up
A general characteristic of material objects or phenomena, which can acquire an individual value for each of them, is called a physical quantity.
To measure a physical quantity means to compare it with a homogeneous quantity taken as a unit.
As a result of measurements, we obtain the value of physical quantities.
Speaking about the value of a physical quantity, one should indicate its numerical value and unit.
Measuring instruments are used to measure physical quantities.
To reduce the notation of numerical values of large and small physical quantities, multiples and submultiples are used. They are formed with the help of prefixes.
- test questions
1. Define a physical quantity. How do you understand it?
2. What does it mean to measure a physical quantity?
3. What is meant by the value of a physical quantity?
4. Name all the physical quantities mentioned in the passage from the novel by J. Verne, given in the text of the paragraph. What is their numerical value? units?
5. With the help of what prefixes are submultiple units formed? multiple units?
6. What characteristics of the device can be set using the scale?
7. What is called the division price?
- Exercises
1. Name the physical quantities known to you. Specify the units of these quantities. What instruments are used to measure them?
2. In fig. 1.22 shows some measuring instruments. Is it possible, using only the figure, to determine the value of division of the scales of these devices. Justify the answer.
3. Express in meters the following values of the physical quantity: 145 mm; 1.5 km; 2 km 32 m.
4. Write down the following values of physical quantities using multiples or submultiples: 0.0000075 m - the diameter of red blood cells; 5,900,000,000,000 m is the radius of the orbit of the planet Pluto; 6,400,000 m is the radius of the planet Earth.
5 Determine the limits of measurement and the division value of the instrument scales that you have at home.
6. Recall the definition of a physical quantity and prove that length is a physical quantity.
- Physics and technology in Ukraine
One of the outstanding physicists of our time - Lev Davidovich Landau (1908-1968) - demonstrated his abilities while still in high school. After graduating from university, he trained with one of the creators of quantum physics, Niels Bohr. Already at the age of 25, he headed the theoretical department of the Ukrainian Institute of Physics and Technology and the Department of Theoretical Physics of Kharkov University. Like most outstanding theoretical physicists, Landau had an extraordinary breadth of scientific interests. Nuclear physics, plasma physics, the theory of superfluidity of liquid helium, the theory of superconductivity - Landau made a significant contribution to all these branches of physics. He was awarded the Nobel Prize for his work on low temperature physics.
Physics. Grade 7: Textbook / F. Ya. Bozhinova, N. M. Kiryukhin, E. A. Kiryukhina. - X .: Publishing house "Ranok", 2007. - 192 p.: ill.
Lesson content lesson summary and support frame lesson presentation interactive technologies accelerating teaching methods Practice quizzes, testing online tasks and exercises homework workshops and trainings questions for class discussions Illustrations video and audio materials photos, pictures graphics, tables, schemes comics, parables, sayings, crossword puzzles, anecdotes, jokes, quotes Add-ons abstracts cheat sheets chips for inquisitive articles (MAN) literature main and additional glossary of terms Improving textbooks and lessons correcting errors in the textbook replacing obsolete knowledge with new ones Only for teachers calendar plans training programs methodological recommendations 3. Frontal survey- Guys, what concepts did we meet in the last lesson?
- At home, it was necessary to draw a table in a notebook in which it was necessary to distribute the following words into columns (physical body, substance, phenomenon): lead, thunder, rails, snowstorm, aluminum, dawn, snowstorm, moon, alcohol, scissors, mercury, snowfall, table, copper, helicopter, oil, boiling, snowstorm, shot, flood.
Completion of the table is checked orally.
In the meantime, one student draws up on the board the solution to the task of converting units of measurement.
After that, the children themselves evaluate the correctness of the completed task.
The most active students who commented and answered confidently, correctly and reasonedly should be evaluated.
- The third task was creative: to pick up riddles about physical bodies, phenomena, substances.
Let's play the chain game. The condition of the game is as follows: I will give you a riddle, and you not only have to guess it, but also determine: a body, a substance or a phenomenon. Whoever guesses, he reads his own. Whoever guesses the riddle of a classmate, he offers his own, etc. along the chain. And the last condition: the riddles are not repeated.
Mystery:
Miracle bird, scarlet tail
Flew into a flock of stars.
- Well done!
Evaluation of the results of homework.
Marked in the journal.
The design of a creative task in the form of puzzles, crossword puzzles, drawings is welcome.
4. Learning new material
- Guys, how long do you think it took us to check our homework?
– Have you ever had to take measurements in everyday life? Which?
– All of these listed examples are physical quantities. Today in the lesson we will get to know them in more detail and learn how to measure them. ( slide 1).
- Write down in your notebook the date and the topic of the lesson: "Measurement is the basis of technology."
What measuring instruments do you know? What quantities can be measured with their help? ( slide 2)
- You know a lot of physical devices!
- Do you know how, with their help, to determine the magnitude?
- Shall we check?
I will divide you into groups of 5 people. And each group will experimentally test and confirm their knowledge.
The class is divided into 5 groups with an equal number of children, but different skills and abilities. Since the groups are multi-level, therefore, it is necessary to select differentiated tasks: low, medium, high level. ( Appendix 3
)
When performing the experiment, I remind you of the basic safety rules: working with thermometers, with small objects and with sharp objects.
The speaking student (from each group) is evaluated, and the correctness of homework is also taken into account.
- Well done!
“You have all now proved that you know how to use measuring instruments.
- Tell me, why do we need to know the length and width of the palm?
Why do we need to know how to determine body weight?
– Where and when did you measure the temperature?
- When else can we measure the volume of the body, using a ruler?
- Guys, think about how you can determine the volume of air in the classroom?
Let's write this formula in a notebook.
How to determine the volume of a piece of chalk? (showing crayon).
- But we are surrounded not only by bodies with the correct geometric shape. For example, porcelain roller, Kinder-surprise toy, spoon, etc.
All items are on display.
- How to determine the volume of an irregularly shaped body? For example, Kinder-surprise toys?
- The volume of a small toy is measured with a physical device - a beaker.
- Write down the name of this device in your notebook.
- How to measure the volume of the body with a beaker? To do this, a certain amount of water is poured into the beaker. The body to be examined is completely immersed in a beaker of water and the water level is noticed to have increased. The difference in the readings of the volumes of water will be the desired value - the volume of the body.
- Write down the formula in your notebook:
V \u003d V 1 - V 2, where V 1 is the volume of water in the beaker, and V 2 is the volume of water and the body immersed in it.
- Who will determine the volume of a copper cylinder using a beaker?
It is necessary to take into account the following: this experiment is visible only near the seated audience. Therefore, it is demonstrated slide 3(the result of the experiment).
- Guys, what do all measuring instruments have in common? ( Slide 2. Hyperlink).
Next, follow the hyperlink to slide 4. Scale and its characteristics.
- Let's consider the same device for its intended purpose, but with different scales. On page 9 of the textbook fig. 11 and 12.
- Guys, tell me if the thermometer readings are the same.
What thermometer shows the highest temperature?
- In order to accurately be able to take readings from the device, you need to know its division value.
- Write in your notebook the subtitle "Price of division."
– The division value is the smallest value of a physical quantity that the device can measure.
- In order to correctly determine the price of the division, there is a rule. ( slide 5) We find the same rule in the textbook.
We are learning to determine the price of division of the beaker scale. ( slide 6).
– Write down the formula for determining the division value:
C \u003d (a - b) / d. ( Slide 7).
We learn to determine the price of division of the scale and measure instrument readings. ( Slides 8, 9).
5. Consolidation of the studied material
- Well done!
- Guys, what new did you learn at the lesson today?
Evaluation of those children who were active in the lesson, taking into account the work in the group.
6. Homework
- Write your homework in your diary. ( Slide 10).
I distribute cards with tasks of two options. ( Appendix 4
)
I answer the questions of the children, if they arose during the acquaintance with the tasks.
In the next lesson, students check this work with each other and mark it in the margins with a pencil.
– In the remaining time, we will play “Catch Me”. ( slide 11)
- The condition of the game: I ask leading statements, and your task is to guess what is being discussed as early as possible. If the answer is correct, a guess will appear on the screen.
What physical quantity can be measured with their help?
– Where else is this device used?
- The second riddle. ( slide 12).
Where and what is this device used for?
– Third riddle :( slide 13).
– Have you met this device and where?
The most savvy also needs to be evaluated.
- Well done, thank you all for your attention. Thank you all very much. ( Slide 14).
Solar radiation meter (luxmeter)
To help technical and scientific staff, many measuring instruments have been developed to ensure accuracy, convenience and efficiency. At the same time, for most people, the names of these devices, and even more so the principle of their operation, are often unfamiliar. In this article, we will briefly reveal the purpose of the most common measuring instruments. Information and images of devices were shared with us by the website of one of the suppliers of measuring devices.
Spectrum analyzer- This is a measuring device that serves to observe and measure the relative distribution of energy of electrical (electromagnetic) oscillations in a frequency band.
Anemometer- a device designed to measure the speed, volume of air flow in a room. The anemometer is used for sanitary and hygienic analysis of territories.
Balometer– a measuring device for direct measurement of the air volume flow on large supply and exhaust ventilation grilles.
Voltmeter is a device that measures voltage.
Gas analyzer- a measuring device for determining the qualitative and quantitative composition of gas mixtures. Gas analyzers are either manual or automatic. Examples of gas analyzers: freon leak detector, hydrocarbon fuel leak detector, particulate number analyzer, flue gas analyzer, oxygen meter, hydrogen meter.
Hygrometer is a measuring device that serves to measure and control the humidity of the air.
Rangefinder- a device that measures distance. The rangefinder also allows you to calculate the area and volume of an object.
Dosimeter- a device designed to detect and measure radioactive emissions.
RLC meter- a radio measuring device used to determine the total conductivity of an electrical circuit and impedance parameters. RLC in the name is an abbreviation of the circuit names of elements whose parameters can be measured by this device: R - Resistance, C - Capacitance, L - Inductance.
Power meter- a device that is used to measure the power of electromagnetic oscillations of generators, amplifiers, radio transmitters and other devices operating in the high-frequency, microwave and optical ranges. Types of meters: absorbed power meters and transmitted power meters.
THD meter- a device designed to measure the coefficient of non-linear distortion (coefficient of harmonics) of signals in radio engineering devices.
Calibrator- a special standard measure that is used for verification, calibration or graduation of measuring instruments.
Ohmmeter, or resistance meter is a device used to measure the resistance to electric current in ohms. Varieties of ohmmeters depending on sensitivity: megaohmmeters, gigaohmmeters, teraohmmeters, milliohmmeters, microohmmeters.
Current clamp- a tool that is designed to measure the amount of current flowing in a conductor. Current clamps allow you to measure without breaking the electrical circuit and without disturbing its operation.
thickness gauge- is a device with which it is possible with high accuracy and without violating the integrity of the coating, to measure its thickness on a metal surface (for example, a layer of paint or varnish, a layer of rust, a primer, or any other non-metallic coating applied to a metal surface).
Luxmeter- This is a device for measuring the degree of illumination in the visible region of the spectrum. Light meters are digital, highly sensitive devices such as luxmeter, brightness meter, pulse meter, UV radiometer.
pressure gauge- a device that measures the pressure of liquids and gases. Types of pressure gauges: general technical, corrosion-resistant, pressure gauges, electrocontact.
multimeter- This is a portable voltmeter that performs several functions at the same time. The multimeter is designed to measure DC and AC voltage, current, resistance, frequency, temperature, and also allows you to carry out continuity and diode testing.
Oscilloscope- This is a measuring device that allows you to monitor and record, measure the amplitude and time parameters of an electrical signal. Types of oscilloscopes: analog and digital, portable and desktop
Pyrometer is a device for non-contact temperature measurement of an object. The principle of operation of the pyrometer is based on measuring the thermal radiation power of the measurement object in the range of infrared radiation and visible light. The accuracy of temperature measurement at a distance depends on the optical resolution.
Tachometer- This is a device that allows you to measure the speed of rotation and the number of revolutions of rotating mechanisms. Types of tachometers: contact and non-contact.
Thermal imager- This is a device designed to observe heated objects by their own thermal radiation. The thermal imager allows you to convert infrared radiation into electrical signals, which in turn, after amplification and automatic processing, are converted into a visible image of objects.
Thermohygrometer is a measuring device that simultaneously measures temperature and humidity.
Road defect detector- This is a universal measuring device that allows you to determine the location and direction of cable lines and metal pipelines on the ground, as well as determine the location and nature of their damage.
pH meter is a measuring device designed to measure the hydrogen index (pH index).
Frequency meter– a measuring device for determining the frequency of a periodic process or the frequencies of the harmonic components of the signal spectrum.
Sound level meter- a device for measuring sound vibrations.
Table: Units of measurement and designations of some physical quantities.
Noticed an error? Select it and press Ctrl+Enter
Every person who works in a certain field of activity is faced with measuring instruments. With their help, you can measure certain indicators and measure different objects.
You can buy such devices here, where they are available in a huge assortment. The accuracy of the result that you get in the end depends on the quality of the measuring device.
Determining the scale division value
A certain value, which is called the scale division price, is calculated according to certain rules.
Here are the main points to keep in mind:
- at the very beginning, you need to take those values of the scale that are located in the neighborhood;
- then it is necessary to calculate their difference;
- after that, consider the number of intermediate divisions that are located between the same values;
- at the very end, the resulting difference is divided by the number of intermediate divisions.
These are the main steps that will allow you to determine the price of the division of the scale. If you did it right, you can get the most accurate result.
Such devices have advantages that distinguish them favorably from other options. Measuring devices are stable, able to last as long as possible, show the result with the highest accuracy.
Professionals who work in different fields of activity often use multifunctional devices. With the help of such equipment, it is possible to measure simultaneously according to different indicators.
Modern measuring devices allow you to store data in memory and sort them into archives. If in the future you need to return to past information, then you will extract it and carefully review it.
Measuring instruments have other advantages as well. For example, one device replaces several models at once.
It will be convenient for you to use such equipment, because it is very easy to carry it from place to place. You will have free hands, so you won't drop or break anything.
Main types of measuring equipment
You can use a rangefinder to measure different distances. This is a laser tool that accurately determines the depth of the well and the length of the load-bearing wall.
To get the most accurate leveling result, you need to purchase an optical level. This device is able to solve many tasks and problems.
You can line up lines, apply markings or project different planes with the help of a laser plane builder. Such a tool is indispensable during repairs or complex construction work.
Physical quantities. Measurement of physical quantities. We look at the accuracy and error of measurements in the video:
Current clamps are a device, the main purpose of which is to measure electric current without breaking the electrical circuit and disrupting its functioning.
Additionally, this device is also capable of measuring voltage, frequency, temperature (in some models).
In accordance with the measured values, they are divided into ammeters, voltmeters, wattmeters, phase meters, ampervoltmeters.
The most common are clamp-on ammeters for measuring alternating current, called current clamps. With their help, you can quickly measure the current in the conductor without breaking or disconnecting the electrical circuit. Electrical clamps can be used in electrical installations up to 10000V.
Any layman knows about the purpose of many electrical appliances and tools - everyone knows why a soldering iron or an electric drill is needed. But not everyone, not even every enterprise, has clamp meters.
Despite this, current clamps are intended for widespread use, it's just that many people do not know about the existence of such a device and do not know how to use it.
Where are electrical clamps used?
Current clamps can become an indispensable tool for both household consumers and enterprises of various sizes. With their help it is possible:
- - to determine the actual load in the network. To determine the load of a single-phase network, a measurement is made on the input cable, the resulting current value in amperes is multiplied by the voltage in the network and the cosine of the angle between the phases (cos φ). If there is no reactive load (powerful inductive elements, chokes, motors), then the last value is taken equal to one (cos φ = 1).
- - to measure the power of various devices. If necessary, the current strength of the circuit section with the connected consumer is measured. Power is determined by the above formula.
- - to check the functioning of metering devices for electricity consumption, for example, reconciliation of meter readings with actual consumption.
Construction and notation
Part electrical clamps any modification includes the following main parts: magnetic clamp pliers, range and function switch, display, output connectors, measurement fixation button. This article discusses current clamps of the brand matech M266.
The switch can be set to one of the positions of the measurement modes:
- - DCV – direct voltage;
- - ACV - alternating voltage;
- - DCA - direct current;
- - ACA - alternating current;
- - Ω - resistance;
- - diode icon – diode test;
- - signal icon - a dial tone with a buzzer.
Three input sockets of the device have overload protection. When connecting the device, the black wire of the probes is connected to the “COM” connector, and the red one to the “VΩ” connector. The third connector, labeled "EXT", is used to connect an insulation meter.
Current measurement procedure
The limit switch is set to the desired AC measurement range. Current clamps are connected to the measured conductor.
If the display shows only the value "1", then the limit switch must be set to a higher value, since an overload has occurred.
Voltage measurement procedure
Connect the red wire of the probe to the “VΩ” connector, the black one to “COM”. Set the limit switch to the position corresponding to the measured range.
Connect the probes to the measured load or voltage source. On the screen of the device, the measured voltage will be observed, as well as its polarity. If only the value "1" is observed on the screen, then the limit switch must be switched to a higher value, as an overload has occurred.
Resistance measurement procedure
The probes of the device are the same as when measuring voltage. Set the range switch to the "Ω" range. If the device is used for dialing, then the switch must be set to the appropriate position. If the resistance of the measured section of the circuit is less than 50 ohms, then the buzzer will sound.
Clamp meters - principles of operation
The operation of the simplest AC current clamps is based on the principle of a single-turn current transformer.
Its primary winding is nothing more than a wire or bus in which the current is measured. The secondary winding, which has a larger number of turns, is wound on a detachable magnetic circuit and is located in the clamps themselves. An ammeter is connected to the secondary winding.
By measuring the current that flows in the secondary winding, taking into account the known transformation ratio of the instrument transformer, it is possible to obtain the value of the current measured in the conductor.
Note that with clamp meters measuring the current (and in fact - the load) in the circuit is not at all difficult and very convenient. The measurement process itself is as follows.
The measured value is set using the knob. The tongs are opened, a conductor is passed through them, the handle is released and the tongs are closed. The further procedure for using electrical clamps is exactly the same as when handling a conventional tester.
You can connect clamps to both insulated and bare wires. Most importantly, only one bus should be covered. The indicator of the device displays the current value of the measured circuit.
Thus, if you cover the conductor and press the button, then after opening the magnetic circuit, the fixed measured reading of the device will be stored on the instrument screen.
An alternating current passes through the current-carrying part, which is covered by a magnetic circuit. An alternating magnetic flux is created in the magnetic circuit, as a result of which electromagnetic induction occurs in the secondary winding - a current begins to flow through it (secondary winding), which is measured by an ammeter.
Modern clamp meter are performed according to a scheme that combines a current transformer and a rectifier. It allows the secondary leads to be connected to the meter through a set of shunts rather than directly.
How to use clamp meters
How to measure the network load in the apartment?
The range switch is set to the ACA 200 position. Having opened the current clamps, cover the insulated wire with them at the entrance to the apartment, fix the readings that appeared on the device screen.
The resulting value is multiplied by the mains voltage of 220 V, the cosine is taken equal to one.
Example. Let's say the device shows 6A. This means that the load of the apartment network is:
P \u003d 6 220 \u003d 1320 W \u003d 1.32 kW.
Based on these data, it is possible to check the correct operation of the consumed electricity meter, compliance with the actual load of the input cable, etc.
A little measurement trick
How can a small current be measured with a clamp meter?
In order to measure clamp meters a small current strength, it is necessary to wind the wire on which you need to know the current several times on an open magnetic circuit. Set the measurement limit to the minimum value.
In order to determine the actual value of the current, it is necessary to divide the readings of the device by the number of turns of wire wound on the magnetic circuit.
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