The importing thing is knowing what you are measuring.
The greatest example of measuring, or at least detecting a magnetic field, was the graphite, paper, magnet trick. Remember when you see this, you are not seeing lines of force directly but the result of the interaction between the highly susceptible graphite and the magnetic field flux.
An event happened that demonstrated to me that the term rotated means something and he is right to a point but one must understand the difference between surface magnetics, internal magnetics, and super structure magnetics. What the heck does the term rotating magnetics mean?
There are several terms which rotating can be associated. One is the direct rotation of the magnetic field flux with respect to a point and the other is twisting, but better called rotating magnetic flux vector. At the vector point is a tensor field.
Vanja Janezic, wrote me and used the quote; "the magnetic field is like a shadow, the disk turns but the field doesn't." And that is sort of how it is for part of the earth's magnetic field. Having read that, I tried to explain a tensor field better. TENSOR FIELD IDEALIZED.
Magnetic measurements can be easy or very difficult.
For example; making a general directional magnetic measurement on the Earth's surface away from the poles simply requires a magnetic pointer suspended in space with very little counter torque such that the pointer can align itself with the magnetic field. This is a compass, which has been around for a long time and is easily made. Then there is the sophisticated magnetic measurements of the Sun's surface using Zeeman effect, Hanle effect, and Faraday rotation, all of which require an emission of some type, either spectral lines or radio waves. These techniques are relatively new, they require expensive instrumentation and are somewhat unreliable. Of course, there is the problem of measuring the magnetic properties of Pluto. Since there is no emission of any type, and putting a magnetometer on the planet is extremely difficult, there is no measurements of the magnetic properties of Pluto.
A magnetometer is generally any instrument for measuring magnetic direction and intensity. However, the compass is a magnetometer, and it can't measure intensity, only direction. A magnetometer measures a magnetic field at the location of the magnetometer. It is not a remote sensing device. There are many types of magnetometers; astatic, vibrating mechanical, vibrating coil, pendulum, reed, flux-gate, hall-effect, nuclear resonance, alkali-vapor, metastable-helium and the good old compass. Some of these devices can measure very slight magnetic fields with reasonable accuracy, and others make a real general measurement like the compass. Using two magnetometers combined into one device it is possible to measure both the horizontal and vertical magnetic field.
All the magnetic measurement devices are strictly limited to measuring the vector sum of the magnetic fields at any given point. In other words, if a bar magnetic were placed on a table and a magnetometer were placed 'X' distance from the bar magnet, such that the Earth's magnetic field affected the magnetometer reading equally as much the bar magnet, the magnetometer would only yield the total vectored intensity and direction, and there would be no measurement of the individual magnetic components of the bar magnetic and Earth's magnetic field.
Since most the measurements of the Sun's magnetic field are done by the Zeeman effect, it is reasonable to discuss the effect. The Zeeman effect is the measurement of the splitting spectral emission lines in the presence of a magnetic field. The effect is caused by the torque upon a spinning electron at the time the electron changes state and emits a photon. Many difficulties and errors can occur. Calibration is very difficult and depends upon the magnetic gradient across the solar surface. Some vector magnetographs have problems with the Q and V Stokes parameters causing, serious signal mixing errors. The horizontal fine structure of the magnetic field can influence measurements and possibly yield the wrong direction of the magnetic field vector. There are some drastic vertical variation errors. The other remote magnetic measuring devices as the Hanle affect, Gyro-Sychrotron Radiation and Faraday Rotation magnetometers, generally have as many possible errors as the Zeeman effect devices. So to put this in proper prospective, remote sensor magnetometers are not really accurate.
Remote magnetic measuring devices also gives rise to "What's being measured?" With the Sun's magnetic measurements, only those magnetic fields near the light emitting source are detected. This does not necessarily mean that the magnetic structure of the Sun at distances away from the surface are accurately portrayed.
They do however, give a general picture of the magnetic properties of the surface of the Sun and of Jupiter. Jupiter being a radiator of radio waves some general magnetic effects can be observed using a remote sensor.
The properties of the Earth's magnetic field are more easily measured.
Because there are places where the Earth's magnetic field are not like a dipole a system of measurements was developed to include the horizontal and vertical components.
However, now with better measurement devices it has been found that not only is there horizontal and vertical fields but, also perpendicular fields. Or to put this in other words, latitude, longitude, in and out field components.
Unlike studying other astro bodies it is possible to study the Earth's magnetic history. This is done by measuring the magnetic properties of materials which were heated and cooled during the Earth's history. In recent history man did this by cooking in ovens and pits. Ancient history is measured by looking at volcanoes. Both types of sites were heated such that the hysteresis effect of the material is lost and then cooled to where the hysteresis of the material retains the magnetic information of the time of cooling. The term is called paleomagnetology.
The study of the history of the Earth's magnetic field shows clearly that the magnetic field is not as stable as was previously thought. It is also now known that the Earth's magnetic field is constantly changing. The north and south poles are changing at different rates and in different directions at the same. There are several major magnetic observatories on the Earth's surface, and the magnetic field's polarities are different for each of them. In other words, the north pole to one observatory is not the same as the north pole to another observatory.
Generally, the dipole moment of the Earth is 8.2 X10^25 G /cm^3.
In all my research on the Earth's magnetic field, one point seems to be missed. That point is about the direction of magnetic flux movement. To my knowledge, at this time there is no measured data of the Earth's magnetic flux movement. Now, this doesn't mean that the measurements haven't been done, only that I haven't found any.
First one may ask "Why is it important to know if the flux is moving." Then "What is a moving magnetic flux any way?"
To explain a moving flux, one must look at a uniform magnetic flux with a conductor moving relative to the flux within it. If the conductor is moving perpendicular to a uniform flux at a constant speed, an electric field is generated in the conductor. But wait, what is moving? Is the conductor moving or the flux? What is the affect if the conductor and the flux are both move the same speed and direction relative to each other? The answer to that is "there is no affect." In the above tensor example, it is the difference from being inside the magnet or outside it. The Earth is a sphere spinning in space. The Earth's magnetic flux is largely uniform about sphere. A compass will point north whether the magnetic field is moving relative to the surface or not.
The reason why magnetic flux movement is important is it will tell if the magnetic field is generated by the Earth or a result of outside influence which the Earth is spinning through. Personally, I believe it will be found that there is both a fixed field and moving field. Meaning there is a magnetic field flux moving with you caused by the close proximity to the surface, and a more fixed field associated with the tangential motion of a day, and yet another one dependent on where the earth is relative to the year, and yet even another based on where the sun is relative to the galaxy.
An experiment can be performed to find the answer. All that needs to be done is string a long wire perpendicular to the field and measure the electric field generated as the Earth turns. Three angles would give a more complete picture of the magnetic components. If no field is generated, then the magnetic field moves with the Earth. If an electric field is generated, then the magnetic field is fixed with respect to the Earth.
Like Faraday's paradox.
The term magnetic insulator is use. This can be several things: a small area of inductance compared to a large area, inside a device where the field is far more susceptible to go around the area sensed than through it, or a line miles long some 60 feet from high power transmission lines and a the return line some quarter mile a way. Any uneven magnetic field changing in time where the field magnetic intensity is different between the drive and return line will cause induction. Area is a function of current. Put the system under an appropriate load for the difference in force, and a measurement can be made.
Practically, to measure the electric field requires a voltmeter reading both ends of a conductor perpendicular to the field. The volt meter leads must be insulated from the magnetic field or they will also generate a field. If there is no motion of the magnetic field with respect to the Earth's spin velocity, the approximate velocity is 
* 12,756,000 / 86,400 = 50 meters/second. The approximate distance = 12,000 meters. The approximate magnetic flux B = .00003 webers / square meter, & emf = - B * l * v. Which should yield 18 volts assuming a good magnetic insulator. Any variations in the voltage would indicate either variations in field intensity or field motion. The most ideal place to do this measurement is Monaloa Hawaii.
Then there is the argument if you can sense it you should be able to get power from it. And, most assuredly you can. The trade off is the cost. Faraday's paradox would work at the poles of the earth. But...It would take a lot of copper. Idealism be damned, it would work better with gold. .00003 webers / square meter, is next to nothing when one is faced with developing energy. Energy being volts and amperes for a length of time, generally called watt-hours. Most the magnetics associated with major power generators are 10,000 to 100,000 gauss. The Earth's pole flux intensity is about 6 gauss which isn't much when the generation of energy is concerned.
The reason this measurement would seem important is because one could subtract the two field measurements and come up with what is moving and what is not regarding the Earth's magnetic field. All we seem to have today is a vector XYZ amount at any given location in a day, and that just isn't enough. However, the XYZ measurements seem to verify the theory that the Earth's magnetic flux varies with the time of day.
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