Let's understand together what a magnetic field is. After all, many people live in this field all their lives and do not even think about it. Time to fix it!
A magnetic field
A magnetic field – special kind matter. It manifests itself in the action on moving electric charges and bodies that have their own magnetic moment (permanent magnets).
Important: a magnetic field does not act on stationary charges! A magnetic field is also created by moving electric charges, or by a time-varying electric field, or magnetic moments electrons in atoms. That is, any wire through which current flows also becomes a magnet!
A body that has its own magnetic field.
A magnet has poles called north and south. The designations "northern" and "southern" are given only for convenience (as "plus" and "minus" in electricity).
The magnetic field is represented by force magnetic lines. The lines of force are continuous and closed, and their direction always coincides with the direction of the field forces. If metal chips are scattered around a permanent magnet, the metal particles will show a clear picture. lines of force magnetic field, leaving the northern and entering South Pole. Graphical characteristic of the magnetic field - lines of force.
Magnetic field characteristics
The main characteristics of the magnetic field are magnetic induction, magnetic flux and magnetic permeability. But let's talk about everything in order.
Immediately, we note that all units of measurement are given in the system SI.
Magnetic induction B – vector physical quantity, which is the main power characteristic of the magnetic field. Denoted by letter B . The unit of measurement of magnetic induction - Tesla (Tl).
Magnetic induction indicates how strong a field is by determining the force with which it acts on a charge. This force is called Lorentz force.
Here q - charge, v - its speed in a magnetic field, B - induction, F is the Lorentz force with which the field acts on the charge.
F- a physical quantity equal to the product of magnetic induction by the area of the contour and the cosine between the induction vector and the normal to the plane of the contour through which the flow passes. magnetic flux- scalar characteristic of the magnetic field.
We can say that the magnetic flux characterizes the number of magnetic induction lines penetrating a unit area. The magnetic flux is measured in Weberach (WB).
Magnetic permeability is the coefficient that determines the magnetic properties of the medium. One of the parameters on which the magnetic induction of the field depends is the magnetic permeability.
Our planet has been a huge magnet for several billion years. The induction of the Earth's magnetic field varies depending on the coordinates. At the equator, it is about 3.1 times 10 to the minus fifth power of Tesla. In addition, there are magnetic anomalies, where the value and direction of the field differ significantly from neighboring areas. One of the largest magnetic anomalies on the planet - Kursk and Brazilian magnetic anomaly.
The origin of the Earth's magnetic field is still a mystery to scientists. It is assumed that the source of the field is the liquid metal core of the Earth. The core is moving, which means that the molten iron-nickel alloy is moving, and the movement of charged particles is the electric current that generates the magnetic field. The problem is that this theory geodynamo) does not explain how the field is kept stable.
The earth is a huge magnetic dipole. The magnetic poles do not coincide with the geographic ones, although they are in close proximity. Moreover, the Earth's magnetic poles are moving. Their displacement has been recorded since 1885. For example, over the past hundred years, the magnetic pole in southern hemisphere moved almost 900 kilometers and is now in the Southern Ocean. The pole of the Arctic hemisphere is moving across the Arctic Ocean towards the East Siberian magnetic anomaly, the speed of its movement (according to 2004 data) was about 60 kilometers per year. Now there is an acceleration of the movement of the poles - on average, the speed is growing by 3 kilometers per year.
What is the significance of the Earth's magnetic field for us? First of all, the Earth's magnetic field protects the planet from cosmic rays and the solar wind. Charged particles from deep space do not fall directly to the ground, but are deflected by a giant magnet and move along its lines of force. Thus, all living things are protected from harmful radiation.
During the history of the Earth, there have been several inversions(changes) of magnetic poles. Pole inversion is when they change places. Last time this phenomenon occurred about 800 thousand years ago, and there were more than 400 geomagnetic reversals in the history of the Earth. Some scientists believe that, given the observed acceleration of the movement of the magnetic poles, the next pole reversal should be expected in the next couple of thousand years.
Fortunately, no reversal of poles is expected in our century. So, you can think about the pleasant and enjoy life in the good old constant field of the Earth, having considered the main properties and characteristics of the magnetic field. And so that you can do this, there are our authors, who can be entrusted with some of the educational troubles with confidence in success! and other types of work you can order at the link.
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Tasks D13. A magnetic field. Electromagnetic induction
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An electric current was passed through a light conductive frame located between the poles of a horseshoe magnet, the direction of which is indicated by arrows in the figure.
Decision.
The magnetic field will be directed from the north pole of the magnet to the south (perpendicular to the AB side of the frame). The Ampere force acts on the sides of the frame with current, the direction of which is determined by the left hand rule, and the value is . Thus, forces equal in magnitude but opposite in direction will act on the AB side of the frame and the side parallel to it: on the left side “from us”, and on the right side “on us”. The forces will not act on the other sides, since the current in them flows parallel to the field lines of force. Thus, the frame will begin to rotate clockwise when viewed from above.
As it rotates, the direction of the force will change and the moment the frame rotates 90°, the torque will change direction, so the frame will not rotate any further. For some time, the frame will oscillate in this position, and then it will be in the position indicated in Figure 4.
Answer: 4
Source: GIA in Physics. main wave. Option 1313.
An electric current flows through the coil, the direction of which is shown in the figure. At the same time, at the ends of the iron core of the coil
1) magnetic poles are formed: at the end 1 - North Pole; at the end 2 - south
2) magnetic poles are formed: at the end 1 - the south pole; at the end 2 - northern
3) electric charges accumulate: at the end 1 - a negative charge; end 2 - positive
4) electric charges accumulate: at the end 1 - a positive charge; at the end of 2 - negative
Decision.
When charged particles move, a magnetic field always arises. Let's use the rule right hand to determine the direction of the magnetic induction vector: let's direct our fingers along the current line, then the bent thumb will indicate the direction of the magnetic induction vector. Thus, the lines of magnetic induction are directed from end 1 to end 2. The lines of the magnetic field enter the south magnetic pole and exit the north.
The correct answer is numbered 2.
Note.
Inside the magnet (coil), the magnetic field lines go from the south pole to the north.
Answer: 2
Source: GIA in Physics. main wave. Option 1326., OGE-2019. main wave. Option 54416
The figure shows a pattern of magnetic field lines from two bar magnets, obtained using iron filings. Which poles of bar magnets, judging by the location of the magnetic needle, correspond to areas 1 and 2?
1) 1 - the north pole; 2 - south
2) 1 - south; 2 - north pole
3) both 1 and 2 - to the north pole
4) both 1 and 2 - to the south pole
Decision.
Since the magnetic lines are closed, the poles cannot be both south and north at the same time. The letter N (North) denotes the north pole, S (South) - the south. The north pole is attracted to the south. Therefore, area 1 is the south pole, area 2 is the north pole.
In this lesson, the topic of which is: “The magnetic field of a constant electric current”, we will learn what a magnet is, how it interacts with other magnets, write down the definitions of the magnetic field and the magnetic induction vector, and also use the gimlet rule to determine the direction of the magnetic induction vector.
Each of you held a magnet in your hands and knows its amazing property: it interacts at a distance with another magnet or with a piece of iron. What is it about a magnet that gives it these amazing properties? Can you make your own magnet? It is possible, and what is needed for this - you will learn from our lesson. Let's get ahead of ourselves: if we take a simple iron nail, it will not have magnetic properties, but if we wrap it with wire and connect it to a battery, we get a magnet (see Fig. 1).
Rice. 1. A nail wrapped in wire and connected to a battery
It turns out that to get a magnet, you need an electric current - movement electric charge. The properties of permanent magnets, such as fridge magnets, are also associated with the movement of an electric charge. A certain magnetic charge, like an electric one, does not exist in nature. It is not needed, enough moving electric charges.
Before investigating the magnetic field of a direct electric current, it is necessary to agree on how to quantitatively describe the magnetic field. For a quantitative description of magnetic phenomena, it is necessary to introduce the force characteristic of the magnetic field. The vector quantity that quantitatively characterizes the magnetic field is called magnetic induction. It is usually denoted by a capital Latin letter B, measured in Tesla.
Magnetic induction is a vector quantity, which is a force characteristic of a magnetic field at a given point in space. The direction of the magnetic field is determined by analogy with the model of electrostatics, in which the field is characterized by the action on a trial charge at rest. Only here a magnetic needle (an elongated permanent magnet) is used as a "trial element". You saw such an arrow in a compass. The direction of the magnetic field at some point is taken to be the direction that will indicate the north pole N of the magnetic needle after reorientation (see Fig. 2).
A complete and clear picture of the magnetic field can be obtained by constructing the so-called magnetic field lines (see Fig. 3).
Rice. 3. Field lines of the magnetic field of a permanent magnet
These are lines showing the direction of the magnetic induction vector (that is, the direction of the N pole of the magnetic needle) at each point in space. With the help of a magnetic needle, one can thus obtain a picture of the lines of force of various magnetic fields. Here, for example, is a picture of the magnetic field lines of a permanent magnet (see Fig. 4).
Rice. 4. Field lines of the magnetic field of a permanent magnet
A magnetic field exists at every point, but we draw lines at some distance from each other. This is just a way of depicting a magnetic field, similarly we did with the intensity electric field(See Fig. 5).
Rice. 5. Electric field strength lines
The more densely the lines are drawn, the greater the modulus of magnetic induction in a given region of space. As you can see (see Fig. 4), the lines of force exit the north pole of the magnet and enter the south pole. Inside the magnet, the field lines also continue. Unlike electric field lines, which start at positive charges and end at negative charges, magnetic field lines are closed (see Fig. 6).
Rice. 6. Magnetic field lines are closed
A field whose lines of force are closed is called a vortex vector field. The electrostatic field is not vortex, it is potential. The fundamental difference between vortex and potential fields is that the work of the potential field on any closed path is zero, for vortex field this is not true. The earth is also a huge magnet, it has a magnetic field that we detect with a compass needle. Read more about the Earth's magnetic field in the branch.
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Our planet Earth is a large magnet, the poles of which are located near the intersection of the surface with the axis of rotation. Geographically, these are the South and North Poles. That is why the arrow in the compass, which is also a magnet, interacts with the Earth. It is oriented in such a way that one end points to the North Pole, and the other to the South (see Fig. 7).
Fig.7. The arrow in the compass interacts with the Earth The one that points to the North Pole of the Earth was designated N, which means North - translated from English as "North". And the one that points to the South Pole of the Earth - S, which means South - translated from English "South". Since opposite poles of magnets are attracted, the north pole of the arrow points to the South magnetic pole of the Earth (see Fig. 8).
Rice. 8. Interaction of the compass and the magnetic poles of the Earth It turns out that the South magnetic pole is located at the North geographic. Conversely, the North magnetic is located at the South geographic pole Earth. |
Now, having become acquainted with the model of the magnetic field, we examine the field of a conductor with direct current. Back in the 19th century, the Danish scientist Oersted discovered that a magnetic needle interacts with a conductor through which an electric current flows (see Fig. 9).
Rice. 9. Interaction of a magnetic needle with a conductor
Practice shows that in the magnetic field of a rectilinear conductor with current, the magnetic needle at each point will be set tangentially to a certain circle. The plane of this circle is perpendicular to the conductor with current, and its center lies on the axis of the conductor (see Fig. 10).
Rice. 10. The location of the magnetic needle in the magnetic field of a straight conductor
If you change the direction of current flow through the conductor, then the magnetic needle at each point will turn in the opposite direction (see Fig. 11).
Rice. 11. When changing the direction of the flow of electric current
That is, the direction of the magnetic field depends on the direction of current flow through the conductor. This dependence can be described using a simple experimentally established method - gimlet rules:
if direction forward movement gimlet coincides with the direction of the current in the conductor, then the direction of rotation of its handle coincides with the direction of the magnetic field created by this conductor (see Fig. 12).
So, the magnetic field of a conductor with current is directed at each point tangentially to a circle lying in a plane perpendicular to the conductor. The center of the circle coincides with the axis of the conductor. The direction of the magnetic field vector at each point is related to the direction of the current in the conductor by the gimlet rule. Empirically, when changing the current strength and the distance from the conductor, it was found that the modulus of the magnetic induction vector is proportional to the current and inversely proportional to the distance from the conductor. The modulus of the magnetic induction vector of the field created by an infinite current-carrying conductor is equal to:
where is the coefficient of proportionality, which is often found in magnetism. It is called the magnetic permeability of the vacuum. Numerically equal to:
For magnetic fields, as well as for electric ones, the principle of superposition is valid. Magnetic fields created by different sources at one point in space add up (see Fig. 13).
Rice. 13. Magnetic fields from different sources add up
The total power characteristic of such a field will be the vector sum of the power characteristics of the fields of each of the sources. The magnitude of the magnetic induction of the field created by the current at a certain point can be increased by bending the conductor into a circle. This will be clear if we consider the magnetic fields of small segments of such a coil of wire at a point inside this coil. For example, in the center.
The segment marked , according to the gimlet rule, creates an upward field in it (see Fig. 14).
Rice. 14. Magnetic field of the segments
The segment similarly creates a magnetic field at this point directed there. The same is true for other segments. Then the total force characteristic (that is, the magnetic induction vector B) at this point will be a superposition of the force characteristics of the magnetic fields of all small segments at this point and will be directed upwards (see Fig. 15).
Rice. 15. Total power characteristic in the center of the coil
For an arbitrary coil, not necessarily in the shape of a circle, for example, for a square frame (see Fig. 16), the value of the vector inside the coil will naturally depend on the shape, size of the coil and the current strength in it, but the direction of the magnetic induction vector will always be determined in the same way (as a superposition of fields created by small segments).
Rice. 16. Magnetic field of square frame segments
We have described in detail the determination of the direction of the field inside the coil, but in general case it can be found much easier, according to a slightly modified gimlet rule:
if you rotate the handle of the gimlet in the direction where the current flows in the coil, then the tip of the gimlet will indicate the direction of the magnetic induction vector inside the coil (see Fig. 17).
That is, now the rotation of the handle corresponds to the direction of the current, and the movement of the gimlet corresponds to the direction of the field. And not vice versa, as was the case with a straight conductor. If a long conductor, through which current flows, is coiled into a spring, then this device will be a set of turns. The magnetic fields of each turn of the coil will add up according to the principle of superposition. Thus, the field created by the coil at some point will be the sum of the fields created by each of the turns at that point. The picture of the field lines of the field of such a coil you see in Fig. eighteen.
Rice. 18. Power lines of the coil
Such a device is called a coil, solenoid or electromagnet. It is easy to see that the magnetic properties of the coil will be the same as those of a permanent magnet (see Fig. 19).
Rice. nineteen. Magnetic properties coil and permanent magnet
One side of the coil (which is in the picture above) plays the role of the north pole of the magnet, and the other side - the south pole. Such a device is widely used in technology, because it can be controlled: it becomes a magnet only when the current in the coil is turned on. Note that the magnetic field lines inside the coil are nearly parallel and dense. The field inside the solenoid is very strong and uniform. The field outside the coil is non-uniform, it is much weaker than the field inside and is directed in the opposite direction. The direction of the magnetic field inside the coil is determined by the gimlet rule as for the field inside one turn. For the direction of rotation of the handle, we take the direction of the current that flows through the coil, and the movement of the gimlet indicates the direction of the magnetic field inside it (see Fig. 20).
Rice. 20. Rule of the gimlet for the reel
If you place a current-carrying coil in a magnetic field, it will reorient itself like a magnetic needle. The moment of force causing the rotation is related to the modulus of the magnetic induction vector at a given point, the area of the coil and the current strength in it by the following relationship:
Now it becomes clear to us where the magnetic properties of a permanent magnet come from: an electron moving in an atom along a closed path is like a coil with current, and, like a coil, it has a magnetic field. And, as we saw with the example of a coil, many turns of current, ordered in a certain way, have a strong magnetic field.
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The field created by permanent magnets is the result of the movement of charges inside them. And these charges are electrons in atoms (see Fig. 21).
Rice. 21. Movement of electrons in atoms Let us explain the mechanism of its occurrence at a qualitative level. As you know, electrons in an atom are in motion. So, each electron, in each atom, creates its own magnetic field, thus, it turns out great amount magnets the size of an atom. In most substances, these magnets and their magnetic fields are randomly oriented. Therefore, the total magnetic field created by the body is zero. But there are substances in which the magnetic fields created by individual electrons are oriented in the same way (see Fig. 22).
Rice. 22. Magnetic fields are oriented the same Therefore, the magnetic fields created by each electron add up. As a result, a body made of such a substance has a magnetic field and is a permanent magnet. In an external magnetic field, individual atoms or groups of atoms, which, as we found out, have their own magnetic field, turn like a compass needle (see Fig. 23).
Rice. 23. Rotation of atoms in an external magnetic field If before that they were not oriented in one direction and did not form a strong total magnetic field, then after the ordering of elementary magnets, their magnetic fields will add up. And if, after the action of an external field, the order is preserved, the substance will remain a magnet. The described process is called magnetization. |
Designate the poles of the current source feeding the solenoid at the indicated in fig. 24 interactions. Let's reason: a solenoid in which a direct current flows behaves like a magnet.
Rice. 24. Current source
According to fig. 24 shows that the magnetic needle is oriented with the south pole towards the solenoid. Like poles of magnets repel each other, while opposite poles attract. It follows from this that the left pole of the solenoid itself is the north one (see Fig. 25).
Rice. 25. Left pole of the solenoid north
The lines of magnetic induction leave the north pole and enter the south. This means that the field inside the solenoid is directed to the left (see Fig. 26).
Rice. 26. The field inside the solenoid is directed to the left
Well, the direction of the field inside the solenoid is determined by the gimlet rule. We know that the field is directed to the left, so let's imagine that the gimlet is screwed in this direction. Then its handle will indicate the direction of the current in the solenoid - from right to left (see Fig. 27).
The direction of the current is determined by the direction of movement of the positive charge. A positive charge moves from a point with a large potential (the positive pole of the source) to a point with a smaller one (the negative pole of the source). Therefore, the source pole located on the right is positive, and on the left is negative (see Fig. 28).
Rice. 28. Determination of source poles
Task 2
A frame with an area of 400 is placed in a uniform magnetic field with an induction of 0.1 T so that the normal of the frame is perpendicular to the lines of induction. At what current strength will torque 20 act on the frame (see Fig. 29)?
Rice. 29. Drawing for problem 2
Let's reason: the moment of force causing the rotation is related to the modulus of the magnetic induction vector at a given point, the area of the coil and the current strength in it by the following relationship:
In our case, all the necessary data is available. It remains to express the desired current strength and calculate the answer:
Problem solved.
Bibliography
- Sokolovich Yu.A., Bogdanova G.S. Physics: Handbook with examples of problem solving. - 2nd edition redistribution. - X .: Vesta: Publishing house "Ranok", 2005. - 464 p.
- Myakishev G.Ya. Physics: Proc. for 11 cells. general education institutions. - M.: Education, 2010.
- Internet portal "Knowledge Hypermarket" ()
- Internet portal "Unified collection of DER" ()
Homework
All formulas are taken in strict accordance with Federal Institute of Pedagogical Measurements (FIPI)
3.3 A MAGNETIC FIELD
3.3.1 Mechanical interaction of magnets
Near an electric charge, a peculiar form of matter is formed - an electric field. Around the magnet there is a similar form of matter, but it has a different nature of origin (after all, the ore is electrically neutral), it is called a magnetic field. To study the magnetic field, straight or horseshoe-shaped magnets are used. Certain places of the magnet have the greatest attractive effect, they are called poles (north and south). Opposite magnetic poles attract, and like poles repel.
A magnetic field. Magnetic induction vector
For the power characteristic of the magnetic field, the magnetic field induction vector B is used. The magnetic field is graphically depicted using lines of force (magnetic induction lines). Lines are closed, have neither beginning nor end. The place from which the magnetic lines come out is the North Pole (North), the magnetic lines enter the South Pole (South).
Magnetic induction B [Tl]- vector physical quantity, which is the power characteristic of the magnetic field.
The principle of superposition of magnetic fields - if the magnetic field at a given point in space is created by several sources of the field, then the magnetic induction is the vector sum of the inductions of each of the fields separately :
Magnetic field lines. Field line pattern of strip and horseshoe permanent magnets
3.3.2 Oersted's experience. The magnetic field of a current-carrying conductor. The pattern of the field lines of a long straight conductor and a closed ring conductor, a coil with current
A magnetic field exists not only around a magnet, but also around any conductor with current. Oersted's experiment demonstrates the effect of electric current on a magnet. If a straight conductor, through which the current flows, is passed through a hole in a sheet of cardboard, on which small iron or steel filings are scattered, then they form concentric circles, the center of which is located on the axis of the conductor. These circles represent the lines of force of the magnetic field of a current-carrying conductor.
3.3.3 Ampere force, its direction and magnitude:
Amp power is the force acting on a current-carrying conductor in a magnetic field. The direction of Ampère's force is determined by the left hand rule: if left hand positioned so that the perpendicular component of the magnetic induction vector B enters the palm, and four outstretched fingers are directed in the direction of the current, then the thumb bent 90 degrees will show the direction of the force acting on the segment of the conductor with current, that is, the Ampère force.
where I- current strength in the conductor;
B
L is the length of the conductor in the magnetic field;
α is the angle between the magnetic field vector and the direction of the current in the conductor.
3.3.4 Lorentz force, its direction and magnitude:
Since the electric current is an ordered movement of charges, the action of a magnetic field on a current-carrying conductor is the result of its action on individual moving charges. The force exerted by a magnetic field on charges moving in it is called the Lorentz force. The Lorentz force is determined by the relation:
where q is the magnitude of the moving charge;
V- module of its speed;
B is the modulus of the magnetic field induction vector;
α is the angle between the charge velocity vector and the magnetic induction vector.
Please note that the Lorentz force is perpendicular to the speed and therefore it does not do work, does not change the modulus of the charge's speed and its kinetic energy. But the direction of the speed changes continuously.
The Lorentz force is perpendicular to the vectors AT and v, and its direction is determined using the same left-hand rule as the direction of Ampère's force: if the left hand is positioned so that the component of magnetic induction AT, perpendicular to the charge velocity, entered the palm, and four fingers were directed along the movement of a positive charge (against the movement of a negative charge, for example, an electron), then the thumb bent 90 degrees will show the direction of the Lorentz force acting on the charge Fl.
Motion of a charged particle in a uniform magnetic field
When a charged particle moves in a magnetic field, the Lorentz force does no work. Therefore, the modulus of the velocity vector does not change when the particle moves. If a charged particle moves in a uniform magnetic field under the action of the Lorentz force, and its velocity lies in a plane perpendicular to the vector, then the particle will move along a circle of radius R.
"Determination of the magnetic field" - According to the data obtained during the experiments, fill in the table. J. Verne. When we bring a magnet to the magnetic needle, it turns. Graphic representation of magnetic fields. Hans Christian Oersted. Electric field. The magnet has two poles: north and south. The stage of generalization and systematization of knowledge.
"Magnetic field and its graphic representation" - Non-uniform magnetic field. Coils with current. magnetic lines. Ampère's hypothesis. Inside the bar magnet. Opposite magnetic poles. Polar Lights. The magnetic field of a permanent magnet. A magnetic field. Earth's magnetic field. Magnetic poles. Biometrology. concentric circles. Uniform magnetic field.
"Magnetic field energy" - Scalar value. Calculation of inductance. Permanent magnetic fields. Relaxation time. Definition of inductance. coil energy. Extracurrents in a circuit with inductance. Transition processes. Energy density. Electrodynamics. Oscillatory circuit. Pulsed magnetic field. Self-induction. Magnetic field energy density.
"Characteristics of the magnetic field" - Lines of magnetic induction. Gimlet's rule. Rotate along the lines of force. computer model the earth's magnetic field. Magnetic constant. Magnetic induction. The number of charge carriers. Three ways to set the magnetic induction vector. Magnetic field of electric current. Physicist William Hilbert.
"Properties of the magnetic field" - Type of substance. Magnetic induction of a magnetic field. Magnetic induction. Permanent magnet. Some values of magnetic induction. Magnetic needle. Speaker. Modulus of magnetic induction vector. Lines of magnetic induction are always closed. Interaction of currents. Torque. Magnetic properties of matter.
"Motion of particles in a magnetic field" - Spectrograph. Manifestation of the action of the Lorentz force. Lorentz force. Cyclotron. Determination of the magnitude of the Lorentz force. Test questions. Directions of the Lorentz force. Interstellar matter. The task of the experiment. Change settings. A magnetic field. Mass spectrograph. Movement of particles in a magnetic field. Cathode-ray tube.
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