Have you ever wanted to find out how strong a magnet really was, or how the strength of the magnetic field varied as you changed the distance from the magnet. Devices used to measure the local magnetic field are called magnetometers or gaussmeters. There are commercially available meters for this, but they are usually a bit expensive for some experimenting.
The availability of inexpensive hall effect sensors have made is possible to build your own magnetic field meters cheaply. Measure Your Magnetism and Build your own Gaussmeter articles have a nice example circuit using hall effect sensor.
On the left side that 2 kohm trimmer is for zero adjust. The hall sensor output is at around half of the operating voltage when there is no external magnetic field. By setting the trimmer to the same voltage allows you to get exactly 0V reading on the multimeter when there is no magnetic field. Now it is easy to read the magnetic field strength and the polarity from multimeter. When you set the multimeter to millivolts DC range, the reading directly shows you Gauss reading. Just leave out the last digit (millivolt), and you get the the reading in milli-Teslas. Simple and easy. The measurement range is according to datasheet at least +-140 milli-Teslas (1400 Gauss). The adapter can be also used to measure varying magnetic fields by connecting the output to oscilloscope (hall sensor has 20 kHz bandwidth).
We describe one way to make a simple tool that can measure the strength and direction of a strong magnetic field near neodymium magnets, using off-the-shelf components and items you might already have.
When exposed to a magnetic field normal to (into) the flat face of the sensor, the voltage will vary away from that 2.5V midpoint. A conversion table to convert this voltage to a field strength expressed in Gauss looks like this:
While that setup does work, the output is somewhat less than intuitive. One has to do a lot of math in their head to convert the number on the meter to a field strength. It would be better if we could make the voltmeter read zero when there is no magnetic field, and give positive or negative values depending on the direction of the magnetic field.
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Magnets and magnetic fields are used in everyday electrical equipment such as motors and refrigerators. You will also find them in electronic equipment like cell phones and radios. A magnetic field can be produced by a permanent magnet, or by electrical current flowing through a wire. You can make an electromagnet by wrapping a coil of wire around a magnetic material (such as iron, magnesium, or cobalt). When current flows through the coil, a magnetic field is produced. Magnetic fields are also important in communication systems. The waves used to transfer information for television and radio broadcasts or cell phone calls are electromagnetic waves. Light, x-rays, and radio waves are all examples of electromagnetic waves.
A magnetic field can be visualized as magnetic field lines, as shown in Figure 1. The strength of a magnetic field is defined as the density of magnetic field lines and is strongest close to the magnet. The strength of the magnetic field diminishes (lessens) with increasing distance from the magnet.
In general, a device that measures the strength of a magnetic field is called a magnetometer. The official SI unit for magnetic field strength is the tesla (T). Magnetic field strength is also measured in units of gauss (G) (1 G = 10-4 T). A device that measures magnetic field strength in gauss, specifically, is called a gaussmeter. The gaussmeter that you will build for this project is based on the Hall effect, discovered by Dr. Edwin Hall in 1879. Hall discovered that when a current is passing through a thin sheet and a magnetic field is applied perpendicular to the sheet, a voltage (called the Hall voltage) is generated across the third dimension, perpendicular to the direction of the original current. The magnitude of the Hall voltage is proportional to magnetic field strength. The Hall effect is used in different applications, including making an electric motor.
Your gaussmeter will be based on an integrated circuit called a Hall sensor that allows you to measure the Hall voltage generated by a magnetic field. You will measure the voltage using a multimeter. Once you have constructed the gaussmeter, you can use it to measure how the strength of the magnetic field varies with distance from the Hall sensor. How do you expect field strength to vary with distance? Will the relationship be linear or nonlinear?
Radiated emissions testing involves measuring the electromagnetic field strength of the emissions that are unintentionally generated by your product. Emissions are inherent to the switching voltages and currents within any digital circuit, the only question is: how large are the emissions and do they comply with the emissions limits?
The receiving antenna picks up both the signal direct from the EUT, as well as a bounce off the ground. To increase measurement accuracy, the ground is covered with an electromagnetically reflective surface (aluminum, steel, wire mesh etc..) and this ground plane must be relatively flat.
The distance between the antenna and the equipment under test (EUT) is typically 3m, 10m or 30m. The measurement distance is important because you want to ensure that you are measuring the field strength in the far field as opposed to near field.
At 30 MHz, the wavelength is 10m. As you approach the near field or fresnel region (region between near and far field), the electric field may not yet be stable and the measurements will be less accurate.
Some standards mandate a specific separation, while others allow the use of 2 or more different separations. Because the strength of the electromagnetic field varies with distance, the limits are re-calculated at each measurement separation.
Magnetic field testing is a pretty uncommon test, but some standards do mandate it. Magnetic fields are predominantly present at lower frequencies and measurements are usually made with a large loop antenna that is placed very close to the EUT.
If you were determined to do it using EM field measurements, there would need to be a measurable difference in the scans for a working vs non-working board that. The difference would need to be greater than any possible measurement tolerance. Any transducers would suffice including near field probes, current probes, antennas, TEM cells or NF scanners.
A stellar magnetic field is a magnetic field generated by the motion of conductive plasma inside a star. This motion is created through convection, which is a form of energy transport involving the physical movement of material. A localized magnetic field exerts a force on the plasma, effectively increasing the pressure without a comparable gain in density. As a result, the magnetized region rises relative to the remainder of the plasma, until it reaches the star's photosphere. This creates starspots on the surface, and the related phenomenon of coronal loops.
The magnetic field of a star can be measured by means of the Zeeman effect. Normally the atoms in a star's atmosphere will absorb certain frequencies of energy in the electromagnetic spectrum, producing characteristic dark absorption lines in the spectrum. When the atoms are within a magnetic field, however, these lines become split into multiple, closely spaced lines. The energy also becomes polarized with an orientation that depends on orientation of the magnetic field. Thus the strength and direction of the star's magnetic field can be determined by examination of the Zeeman effect lines.
A stellar spectropolarimeter is used to measure the magnetic field of a star. This instrument consists of a spectrograph combined with a polarimeter. The first instrument to be dedicated to the study of stellar magnetic fields was NARVAL, which was mounted on the Bernard Lyot Telescope at the Pic du Midi de Bigorre in the French Pyrenees mountains.
The magnetic field of a rotating body of conductive gas or liquid develops self-amplifying electric currents, and thus a self-generated magnetic field, due to a combination of differential rotation (different angular velocity of different parts of body), Coriolis forces and induction. The distribution of currents can be quite complicated, with numerous open and closed loops, and thus the magnetic field of these currents in their immediate vicinity is also quite twisted. At large distances, however, the magnetic fields of currents flowing in opposite directions cancel out and only a net dipole field survives, slowly diminishing with distance. Because the major currents flow in the direction of conductive mass motion (equatorial currents), the major component of the generated magnetic field is the dipole field of the equatorial current loop, thus producing magnetic poles near the geographic poles of a rotating body.
Another feature of this dynamo model is that the currents are AC rather than DC. Their direction, and thus the direction of the magnetic field they generate, alternates more or less periodically, changing amplitude and reversing direction, although still more or less aligned with the axis of rotation.
The Sun's major component of magnetic field reverses direction every 11 years (so the period is about 22 years), resulting in a diminished magnitude of magnetic field near reversal time. During this dormancy, the sunspots activity is at maximum (because of the lack of magnetic braking on plasma) and, as a result, massive ejection of high energy plasma into the solar corona and interplanetary space takes place. Collisions of neighboring sunspots with oppositely directed magnetic fields result in the generation of strong electric fields near rapidly disappearing magnetic field regions. This electric field accelerates electrons and protons to high energies (kiloelectronvolts) which results in jets of extremely hot plasma leaving the Sun's surface and heating coronal plasma to high temperatures (millions of kelvin). 2b1af7f3a8