The Earth's magnetic field is similar to the field of a dipole placed at the center of a sphere. The magnetic induction vector at points of the magnetic equator is horizontal, and at the magnetic poles it is vertical. At the north pole it is directed downward, at the south pole it is directed upward.
It should be noted that in the northern hemisphere, because... the vector is directed downward, the south pole of the dipole is located, and the north pole is located in the southern hemisphere. (According to the physical essence, in permanent magnets the force of mutual attraction is always experienced by opposite poles). The magnetic field of the northern hemisphere is usually called the northern, and the southern hemisphere - the southern. When moving from one point on the Earth's surface to another, the vector will change in magnitude and direction.
Let's consider the components of the full vector geo tension magnetic field . Let's take a rectangular coordinate system with the origin at the measurement point, the Z axis is directed downward, the X axis is directed to the geographic north, the Y axis is perpendicular to the east; That. Y axis – parallels, X – meridians, angle D – magnetic declination, angle I – magnetic inclination.
Fig.1 Components of the magnetic field
For small values, the anomalous component is a projection onto the direction of the normal field.
,
In such a coordinate system, the projections of the vector onto the directions of the X, Y, Z axes are called, respectively, the northern, eastern and vertical components of the Earth’s magnetic field and are designated by the letters X, Y, Z.
Full vector at most points earth's surface does not coincide with any of the axes.
The projection of a vector onto the XOY plane is called horizontal component magnetic field and denote . The direction of the vector determines the direction of the magnetic meridian, and the plane in which the vectors and lie is called plane of the magnetic meridian.
The angle between the direction of the magnetic meridian at a given point and some given direction is called magnetic azimuth(it is counted from the direction of the magnetic meridian clockwise).
Angle D – between the directions of the geographic and magnetic meridians, is called magnetic declination. It is measured from the X-axis direction in a clockwise direction.
Angle I is between the directions of the vectors and is called magnetic inclination. It is measured from the horizontal plane downwards; in the northern hemisphere the vector is directed downwards, so angle I is positive; in the southern hemisphere it is directed upwards, therefore angle I is negative. The components X, Y, Z, H, D, I are elements terrestrial magnetism . Declinations and inclinations are measured in degrees.
To a first approximation, the Earth's magnetic field can be considered as the field of a ball magnetized along an axis deviating from the axis of rotation by approximately 11.5 degrees. In this case, the magnetic potential of the ball can be defined as a dipole potential.
§ 15. Terrestrial magnetism and its elements. Magnetic cards
The space in which the Earth's magnetic forces operate is called the Earth's magnetic field. It is generally accepted that the magnetic field lines of the earth's field emerge from the south magnetic pole and converge at the north, forming closed curves.The position of the magnetic poles does not remain unchanged; their coordinates slowly change. The approximate coordinates of the magnetic poles in 1950 were as follows:
Northern - φ ~ 76°N; L ~ 96°W;
South - φ ~ 75°S; L ~ 150° O st .
The Earth's magnetic axis is a straight line connecting the magnetic poles, passes outside the center of the Earth, and makes approximately an angle of about 1G.5 with its axis of rotation.
The strength of the Earth's magnetic field is characterized by the intensity vector T, which at any point of the Earth's magnetic field is directed tangent to the lines of force. In Fig. 18 the force of earth's magnetism at point A is depicted by the magnitude and direction of the vector AF. The vertical plane NmAZF, in which the vector AF is located, and therefore the axis of the freely suspended magnetic needle, is called plane of the magnetic meridian. This plane makes an angle RAS with the plane of the true meridian NuAZM, which is called magnetic declination and denoted by the letter d.
Rice. 18.
Magnetic declination d is measured from the northern part of the true meridian to the east and west from 0 to 180°. The eastern magnetic declination is assigned a plus sign, and the western magnetic declination is assigned a minus sign. For example: d=+4°, 6 or d = -11°,0.
The angle NmAF formed by the vector AF with the plane of the true horizon NuAH is called magnetic inclination and is designated by the letter v.
Magnetic inclination is measured from the horizontal plane downwards from 0 to 90° and is considered positive if the northern end of the magnetic needle is lowered, and negative if the southern end is lowered.
Points on the earth's surface at which vector T is directed horizontally form a closed line that crosses the geographic equator twice and is called magnetic equator. Full force terrestrial magnetism - vector T - can be decomposed into horizontal H and vertical Z components in the plane of the magnetic meridian. From Fig. 18 we have:
H = TcosO, Z=TsinO or Z = HtgO.
The quantities d, H, Z and O that determine the Earth’s magnetic field at a given point are called elements of earth magnetism.
The distribution of the elements of terrestrial magnetism over the surface of the globe is usually depicted on special maps in the form of curved lines connecting points with the same value of one or another element. Such lines are called isolines. Equal magnetic declination curves - isogons put isogons on maps (Fig. 19); curves connecting points with equal magnetic voltage are called isodynes, or isodynamics. Curves connecting points of equal magnetic inclination - isoclines, plot isoclines on maps.
Rice. 19.
Magnetic declination - most important element for navigation, therefore, in addition to special magnetic charts, it is indicated on navigational sea charts, on which they write, for example, like this: “Skl. k. 16°.5 W.”
All elements of earth's magnetism at any point on the earth's surface are subject to changes called variations. Changes in the elements of terrestrial magnetism are divided into periodic and non-periodic (or disturbances).
Periodic changes include secular, annual (seasonal) and daily changes. Of these, daily and annual variations are small and are not taken into account for navigation. Secular variations are a complex phenomenon with a period of several centuries. The magnitude of the secular change in magnetic declination varies at different points on the earth's surface in the range from 0 to 0.2-0.3° per year. Therefore, on nautical charts, the magnetic declination of the compass is reduced to a specific year, indicating the amount of annual increase or decrease.
To adjust the declination to the year of navigation, you need to calculate its change over the elapsed time and use the resulting correction to increase or decrease the declination indicated on the map in the navigation area.
Example 18. The voyage takes place in 1968. The compass declination, taken from the map, d = 11°, 5 O st is given to 1960. The annual increase in declination is 5". Reduce the declination to 1968.
Solution. The time period from 1968 to 1960 is eight years; change Ad = 8 x 5 = 40" ~0°.7. Compass declination in 1968 d = 11°.5 + 0°.7 = - 12°, 2 O st
Sudden short-term changes in the elements of the earth's magnetism (disturbances) are called magnetic storms, the occurrence of which is determined by the northern lights and the number of sunspots. At the same time, changes in declination are observed in temperate latitudes up to 7°, and in polar regions - up to 50°.
In some areas of the earth's surface, the declination differs sharply in magnitude and sign from its values at adjacent points. This phenomenon is called a magnetic anomaly. Marine maps indicate the boundaries of magnetic anomaly areas. When sailing in these areas, you must carefully monitor the operation of the magnetic compass, as the accuracy of the operation is impaired.
MINISTRY OF COMMUNICATIONS
RUSSIAN FEDERATION
MOSCOW STATE UNIVERSITY
COMMUNICATION ROUTES (MIIT)
Department "Physics-2"
APPROVED
Editorial and publishing
university council
Guidelines
in physics
Works No. 20, 22, 90
Edited by prof. V.A Nikitenko and Assoc. A.P. Pruntseva
MOSCOW–2003
Guidelines for laboratory work in physics. Works No. 20, 22, 90 / Ed. prof. Nikitenko V.A. (No. 22.90), associate professor Pruntsev A.P. (No. 20) - M.: MIIT, 2003. - 25 p.
Guidelines for laboratory work in physics are intended for students of all institutes and faculties of MIIT, served by the Department of Physics-2, and corresponds to the program and curriculum in physics (section “Electrodynamics”).
The methodological instructions were compiled by teachers: senior teacher Gosudareva N.A. (work No. 20), associate professor. Pruntsev A.P. (work No. 22, 90).
When drawing up guidelines for laboratory work No. 20, the description of the corresponding laboratory work in RGOTUPS was used.
Moscow State University of Railways
messages (MIIT), 2003
Work 20 determination of the horizontal component of the earth's magnetic field strength vector
Purpose of the work: Study of the magnetic field of circular current. Familiarization with the basics of the doctrine of terrestrial magnetism.
Devices and accessories: 1. DC source. 2.Rheostat. 3. Ammeter.4. Switch.5. Tangent galvanometer.
Elements of terrestrial magnetism
The earth as a whole is a huge magnet. In the space surrounding the Earth, there is a magnetic field, the lines of force of which are shown in Fig. 1. The north magnetic pole is located at the southern geographic pole, and the southern magnetic pole is located at the northern geographic pole. The earth's magnetic field is directed horizontally at the equator, and vertically at the magnetic poles. At other points on the earth's surface, the earth's magnetic field is directed at a certain angle.
The existence of a magnetic field at any point on the Earth can be established using a magnetic needle. If you hang a magnetic needle N.S. on a thread L(Fig. 2) so that the suspension point coincides with the center of gravity, the arrow will be set in the direction of the tangent to the line of force of the Earth’s magnetic field.
Magnetic meridian plane
To the center of the earth
In the northern hemisphere, the southern end will be directed towards the Earth, and the arrow axis will make an angle of inclination with the horizon
(at the magnetic equator the inclination 
, equals 0). Vertical plane in which the arrow axis is located is called the plane of the magnetic meridian. All planes of magnetic meridians intersect in a straight line N.S., and traces of magnetic meridians on the Earth’s surface are located at the magnetic poles N And S. Since the magnetic poles do not coincide with the geographic ones, the axis of the needle will deviate from the geographic meridian. The angle formed by a vertical plane passing through the axis of the magnetic needle (magnetic meridian) with the geographic meridian is called magnetic declination
(Fig. 2). Vector
the total strength of the Earth's magnetic field can be decomposed into two components: horizontal
and vertical 
.Values of declination and inclination angles, as well as the horizontal component 
vector
will make it possible to determine the magnitude and direction of the total strength of the Earth's magnetic field at a given point. If a magnetic needle can rotate freely only around a vertical axis, then it will be positioned under the influence of the horizontal component of the Earth’s magnetic field in the plane of the magnetic meridian. Horizontal component
, magnetic declination
and mood
called elements of terrestrial magnetism.
Distinguish eastern And western declination (the north pole of the arrow deviates to the right or left of the geographic meridian).
There is inclination northern And southern(the north or south end of the arrow is located above or below the horizontal plane). These two angles are the magnetic coordinates of a given point. For example, for Moscow 
= 8° (eastern declination), 
=70° (northern inclination).
The elements of earth's magnetism change smoothly when moving from one point to another. If disturbances in this smooth change are observed, then they say that a magnetic anomaly is observed in the area. Anomalies are associated with large deposits of magnetic ores, for example, the Kursk magnetic anomaly.
The strength of the Earth's magnetic field is relatively low, however, the presence of terrestrial magnetism manifests itself significantly in a number of geographical and other phenomena. Such phenomena include auroras and the capture of charged particles from outer space into peculiar traps called the Earth's radiation belts.
Some biophysical experiments suggest that the spatial orientation of birds during long-distance seasonal flights is associated with their ability to sense the direction of magnetic field lines.
TERRESTRIAL MAGNETISM, a department of geophysics that studies the earth's magnetic field. Let the magnetic field strength at a given point be represented by the vector F (Fig. 1). The vertical plane containing this vector is called the magnetic meridian plane. The angle D between the planes of the geographic and magnetic meridians is called declination. There are eastern and western declinations. It is customary to mark eastern declinations with a plus sign, and western declinations with a minus sign. The angle I formed by the vector F with the horizon plane is called inclination. The projection H of the vector F onto the horizontal plane is called the horizontal component, and the projection Z onto the vertical line is called the vertical component.
The main instruments for measuring the elements of terrestrial magnetism are currently the magnetic theodolite and various systems of inclinators. The purpose of a magnetic theodolite is to measure the horizontal component of the magnetic field and declination. A horizontally located magnet, capable of rotating about a vertical axis, is installed under the influence of the earth's magnetic field with its axis in the plane of the magnetic meridian. If it is taken out of this equilibrium position and then left to itself, it will begin to oscillate around the plane of the magnetic meridian with a period T determined by the formula:
where K is the moment of inertia of the oscillating system (magnet and frame) and M is the magnetic moment of the magnet. Having determined the value of K from special observations, it is possible to find the value of the product MN from the observed period T. Then a magnet is placed, the oscillation period of which is determined, at a certain distance from another, auxiliary magnet, which also has the ability to rotate about a vertical axis, and the first magnet is oriented so that the center of the second magnet is on the continuation of the magnetic axis of the first. In this case, in addition to H, the auxiliary magnet will also be affected by the magnetic field M, which may. found by the formula:
where B is the distance between the centers of both magnets, a, b,... are some constants. The magnet will leave the plane of the magnetic meridian and become in the direction of the resultant of these two forces. Without changing the relative arrangement of the parts of the installation, find such a position of the deflecting magnet at which the said resultant will be perpendicular to it (Fig. 2). 
By measuring the deviation angle v for this case, it is possible to find the value of the ratio from the relation sin v = f/H. From the obtained values of MH and H/M, the horizontal component H is determined. In the theory of terrestrial magnetism, a unit denoted by the symbol γ, equal to 0.00001 gauss, is common. A magnetic theodolite can be used as a declinator and a device for measuring declination. By aligning the sighting plane with the direction of the magnetic axis of a magnet suspended on a thread, it is brought into coincidence with the plane of the magnetic meridian. To obtain a reading on the circle corresponding to pointing the sighting device to the geographic north, it is enough to point at some object whose true azimuth is known. The difference in the readings of the geographic and magnetic meridians gives the declination value.
Inclinator - a device for measuring I. Modern magnetometry has two types of devices for measuring inclination - pointer and induction inclinators. The first device has a magnetic needle rotating about a horizontal axis placed in the center of a vertical limb. The plane of movement of the arrow is aligned with the plane of the magnetic meridian; in that case in ideal conditions the magnetic axis of the arrow in the equilibrium position will coincide with the direction of the magnetic voltage at a given point, and the angle between the direction of the magnetic axis of the arrow and the horizontal line will give the value I. The design of the induction inclinator is based on ( earth inductor) the phenomenon of induction in a conductor moving in a magnetic field is based. An essential feature of the device is the coil, which rotates around one of its diameters. When such a coil rotates in the earth's magnetic field, no EMF appears in it only if its axis of rotation coincides with the direction of the field. This position of the axis, marked by the absence of current in the galvanometer to which the coil is closed, is measured on a vertical circle. The angle between the direction of the coil's rotation axis and the horizon will be the inclination angle.
The devices mentioned above are currently the most common. Special mention should be made of the Ogloblinsky magnetic theodolite, which determines the value of H/M by the method of H compensation by the magnetic field, for which the oscillation period is determined.
IN lately the so-called electrical methods for measuring H, in which deflections are made not using a deflecting magnet, but using the magnetic field of coils. To achieve the accuracy required from magnetic measurements (0.2-0.02% of full voltage), the operating current is compared with the current from normal elements (compensation using the potentiometer method).
Measurements made at various points on the earth's surface show that the magnetic field varies from point to point. In these changes one can notice some patterns, the nature of which is best understood from consideration of the so-called. magnetic cards (Fig. 3 and 4).
If you draw lines on a topographic basis connecting points of equal values of any element of terrestrial magnetism, then such a map will present a clear picture of the distribution of this element on the ground. Respectively various elements terrestrial magnetism there are maps with various systems isolines. These isolines have special names, depending on what element they represent. Thus, lines connecting points of equal declinations are called isogons (the line of zero declinations is called the agonic line), lines of equal inclinations are isoclines and lines of equal stresses are isodynes. There are isodynamics of the horizontal, vertical components, etc. If you build such maps for the entire surface of the globe, you can see on them following features. In equatorial regions there are highest values horizontal force (up to 0.39 gauss); towards the poles the horizontal component decreases. The opposite nature of changes occurs for the vertical component. The line of zero values of the vertical component is called magnetic equator. Points with zero horizontal force values are called magnetic poles land. They do not coincide with geographical coordinates and have the following coordinates: north magnetic pole - 70.5° N. w. and 96.0° W. d. (1922), south magnetic pole - 71.2° south. w. and 151.0° E. d. (1912). All isogons intersect at the magnetic poles of the earth.
A detailed study of the earth's magnetic field reveals that the isolines are not nearly as smooth as suggested by the overall picture. On each such curve there are curvatures that disrupt its smooth course. In some areas, these curvatures reach such large values that this area has to be isolated magnetically from the overall picture. Such areas are called anomalous, and in them one can observe values of magnetic elements many times higher than normal field. Study magnetic anomalies found out their close connection with the geological structure of the upper parts earth's crust , ch. arr. in relation to the content of magnetic minerals in them, and gave birth to a special branch of magnetometry, which has applied significance and aims to apply magnetometry and measurements to mining exploration. Such anomalous areas, which are already of great industrial importance, are located in the Urals, Kursk District, Krivoy Rog, Sweden, Finland and other places. To study the magnetic field of such areas, special equipment has been developed (Tyberg-Thalen magnetometer, local calvariometers, etc.), which makes it possible to quickly obtain the necessary measurement results. The study of the earth's magnetic field at any one point reveals the fact of changes in this field over time. A detailed study of these temporal variations in the elements of terrestrial magnetism led to the establishment of their connection with the life of the globe as a whole. The variations reflect the rotation of the earth around its axis, the movement of the earth in relation to the sun, and a whole series of cosmic phenomena. The study of variations is carried out by special magnetic observatories, equipped, in addition to precision instruments for measuring elements of the earth's magnetic field, with special installations for continuous recording of temporary changes in magnetic elements. Such instruments are called variometers, or magnetographs, and are usually used to record variations of D, H and Z. A device for recording variations of declination (variometer D, or unifilar) has a magnet with a mirror attached to it, hanging freely on a thin thread. Variations in declination, which consist in rotations of the plane of the magnetic meridian, cause the magnet suspended in this way to rotate. A beam thrown from a special illuminator, reflected from a magnet mirror, produces a moving light spot, which leaves a trace in the form of a curve on photosensitive paper, rolled onto a rotating drum or lowered vertically. A line drawn by a beam reflected from a stationary mirror and time stamps make it possible to use the resulting magnetogram to find the change in D for any moment in time. If you twist the thread, rotating the upper point of its attachment, the magnet will come out of the plane of the magnetic meridian; by properly tightening it, you can put it in a position perpendicular to the original one. In the new equilibrium position, the magnet will be acted upon, on the one hand, by N, and on the other, by the moment of the twisted thread. Any change in the horizontal component will cause a change in the equilibrium position of the magnet, and such a device will note variations in the horizontal component (variometer H, or bifilar, if the magnet is suspended on two parallel threads). These variations are recorded in the same way as changes in declination are recorded. Finally, the third device, which serves to record variations in the vertical component (Lloyd's balance, variometer Z), has a magnet that oscillates, like a balance beam, about a horizontal axis. By properly moving the center of gravity using a movable weight, the magnet of this device is brought to a position close to horizontal, and is usually installed so that the plane of movement of the magnet is directed perpendicular to the plane of the magnetic meridian. In this case, the equilibrium position of the magnet is determined by the action of Z and the weight of the system. A change in the first value will cause some tilt of the magnet, proportional to the change in the vertical component. These changes in inclination are recorded, like the previous one, photographically and provide material for judgments about variations in the vertical component.
If you subject the curves recorded by magnetographs (magnetograms) to analysis, you can find a number of features on them, of which the clearly expressed diurnal variation will be the first to catch your eye. The position of the maxima and minima of the diurnal cycle, as well as their values, vary within small limits from day to day, and therefore, to characterize the diurnal cycle, some average curves are compiled for a certain time interval. In fig. Figure 5 shows the curves of changes in D, H and Z for the observatory in Slutsk for September 1927, on which the daily variation of the elements is clearly visible.
The most visual way to depict variations is the so-called. vector diagram, representing the movement of the end of the vector F over time. Two projections of the vector diagram on the yz and xy planes are given in Fig. 6. From this fig. One can see how the time of year is reflected in the nature of the daily cycle: in the winter months, the fluctuations of magnetic elements are much less than in the summer months. 
In addition to variations due to the diurnal cycle, sharp changes are sometimes noticed on magnetograms, often reaching very large values. Such sudden changes in magnetic elements are accompanied by a number of other phenomena, such as: polar lights in the Arctic regions, the appearance of induced currents in telegraph and telephone lines, etc., and are called magnetic storms. There is a fundamental difference between the variations due to the normal course and those caused by storms. While normal changes occur for each observation point in local time, variations caused by storms occur simultaneously for the entire globe. This circumstance indicates the different nature of variations of both types.
The desire to explain the distribution of elements of terrestrial magnetism observed on the ground surface led Gauss to construct mathematical theory geomagnetism. The study of the elements of terrestrial magnetism since the first geomagnetic measurements has revealed the existence of the so-called. the secular course of the elements, and further development Gauss's theory included, among other tasks, taking into account these secular variations. As a result of the work of Peterson, Neumayer and other researchers, there is now a formula for the potential that takes into account this secular course.
Among the hypotheses proposed to explain the daily and annual cycle of geomagnetic elements, we should note the hypothesis proposed by Balfour-Stewart and developed by Schuster. According to these researchers, in high electrically conductive layers of the atmosphere under thermal action sun rays movements of gas masses occur. The earth's magnetic field induces in these moving conducting masses electric currents, the magnetic field of which manifests itself in the form of daily variations. This theory well explains the decrease in the amplitude of variations in the winter months and clarifies the prevailing role of local time. As for magnetic storms, recent research has shown their close connection with the activity of the sun. The clarification of this connection led to the following currently generally accepted theory of magnetic disturbances. At the moments of its most intense activity, the Sun emits streams of electrically charged particles (for example, electrons). Such a flow, entering the upper layers of the atmosphere, ionizes it and creates the possibility of the flow of intense electric currents, the magnetic field of which is the perturbation that we call magnetic storms. This explanation of the nature of magnetic storms agrees well with the results of the theory of auroras developed by Stoermer.
Since the magnetic and geographic poles of the Earth do not coincide, the magnetic needle indicates the north-south direction only approximately. The plane in which the magnetic needle is installed is called the plane of the magnetic meridian of a given place, and the straight line along which this plane intersects the horizontal plane is called the magnetic meridian. The angle between the directions of the magnetic and geographic meridians is called magnetic declination; it is usually denoted by a Greek letter. Magnetic declination varies from place to place on the globe.
Magnetic declination is called western or eastern, depending on whether the north pole of the magnetic needle deviates to the west () or east () from the plane of the geographic meridian (Fig. 229). The declination measurement scale is from 0 to 180°. Often the eastern declination is marked with a “+” sign, and the western declination with a “-”.
Rice. 229. The position of the magnetic needle relative to the cardinal points: a) in places with eastern magnetic declination; b) in places with western magnetic declination
From Fig. 228 it is clear that the lines of the earth's magnetic field, generally speaking, are not parallel to the surface of the earth. This means that the magnetic induction of the Earth’s field does not lie in the horizon plane of a given place, but forms a certain angle with this plane. This angle is called magnetic inclination. Magnetic inclination is often denoted by the letter . IN different places The Earth's magnetic inclination varies.
A very clear idea of the direction of the magnetic induction of the earth's magnetic field at a given point can be obtained by strengthening the magnetic needle so that it can freely rotate around both the vertical and horizontal axis. This can be done, for example, using a suspension (the so-called gimbal suspension), shown in Fig. 230. The arrow is set in the direction of the magnetic induction of the field.
Rice. 230. A magnetic needle, mounted in a gimbal, is installed in the direction of the magnetic induction of the earth's magnetic field
Magnetic declination and magnetic inclination (angles and ) completely determine the direction of magnetic induction of the earth's magnetic field in a given place. It remains to be determined numeric value this value. Let the plane in Fig. 231 represents the plane of the magnetic meridian of a given location. We can decompose the magnetic induction of the earth's magnetic field lying in this plane into two components: horizontal and vertical. Knowing the angle (inclination) and one of the components, we can easily calculate the other component or the vector itself. If, for example, we know the modulus of the horizontal component, then from a right triangle we find
Rice. 231. Decomposition of the magnetic induction of the earth's magnetic field into horizontal and vertical components
In practice, it turns out to be most convenient to directly measure precisely the horizontal component of the earth's magnetic field. Therefore, most often the magnetic induction of this field in one place or another on the Earth is characterized by the modulus of its horizontal component.
Thus, three quantities: declination, inclination and the numerical value of the horizontal component completely characterize the Earth’s magnetic field in a given place. These three quantities are called elements of the earth's magnetic field.
129.1. The angle of inclination of the magnetic needle is 60°. If a weight of mass 0.1 g is attached to its upper end, the arrow will be set at an angle of 30° to the horizontal. What weight needs to be attached to the top end of this arrow to make the arrow horizontal?
129.2. In Fig. 232 shows an inclinator, or inclination compass, a device used to measure magnetic inclination. It is a magnetic needle mounted on a horizontal axis and equipped with a vertical divided circle for measuring inclination angles. The arrow always rotates in the plane of this circle, but this plane itself can rotate around a vertical axis. When measuring inclination, the circle is set in the plane of the magnetic meridian.
Rice. 232. For exercise 129.2
Show that if the inclinator circle is installed in the plane of the magnetic meridian, then the arrow will be set at an angle to the horizon plane equal to the inclination of the earth's magnetic field at a given location. How will this angle change if we rotate the inclinator circle around a vertical axis? How will the arrow be positioned when the plane of the inclinator circle is perpendicular to the plane of the magnetic meridian? 129.3. How will a compass needle behave when placed above one of the earth's magnetic poles? How will the tilt arrow behave there?
Accurate knowledge of the quantities characterizing the earth's magnetic field for the largest possible number of points on earth is extremely important. It is clear, for example, that in order for the navigator of a ship or airplane to use a magnetic compass, he must know the magnetic declination at every point on his route. After all, the compass shows him the direction of the magnetic meridian, and to determine the course of the ship he must know the direction of the geographical meridian.
Declination gives him the correction to the compass readings that needs to be made to find the true north-south direction. Therefore, since the middle of the last century, many countries have been systematically studying the earth's magnetic field. More than 50 special magnetic observatories distributed throughout the globe systematically conduct magnetic observations day after day.
Currently, we have extensive data on the distribution of the elements of terrestrial magnetism around the globe. These data show that the elements of terrestrial magnetism vary from point to point naturally and are generally determined by the latitude and longitude of a given point.
