Measuring Magnetic Fields
After reading this section you will be able to do the following:
Determine how the direction and intensity of magnetic fields can be found.
Describe how a field indicator and a Hall-effect meter work.
Since it is impractical to measure the actual field strength within the material, all the devices measure the magnetic field that is outside of the material. There are a number of different devices that can be used to detect and measure an external magnetic field. The two devices commonly used in magnetic particle inspection are the field indicator and the Hall-effect meter, which is also called a gauss meter. Pie gauges and shims are devices that are often used to provide an indication of the field direction and strength but do not actually yield a quantitative measure.
Field Indicators
Field indicators are small mechanical devices that utilize a soft iron vane that is deflected by a magnetic field. The X-ray image below shows the inside working of a field meter looking in from the side. The vane is attached to a needle that rotates and moves the pointer for the scale. Field indicators can be adjusted and calibrated so that quantitative information can be obtained. However, the measurement range of field indicators is usually small due to the mechanics of the device. The one shown below has a range from plus 20 gauss to minus 20 gauss. This limited range makes them best suited for measuring the residual magnetic field after demagnetization.
Measuring Magnetic Fields
Hall-Effect (Gauss/Tesla) Meter
Hall-Effect meters can be used to detect small changes in magnetic field strength.A Hall-effect meter is an electronic device that provides a digital readout of the magnetic field strength in gauss or tesla units. The meters use a very small conductor or semiconductor element at the tip of the probe. Electric current is passed through the conductor. In a magnetic field, a force is exerted on the moving electrons which tends to push them to one side of the conductor. A buildup of charge at the sides of the conductors will balance this magnetic influence, producing a measurable voltage between the two sides of the conductor. The presence of this measurable transverse voltage is called the Hall-effect after Edwin H. Hall, who discovered it in 1879.
The magnetic forces on the hall sensor cause the moment of electrons. That current is then read.
The orientation of the hall sensor on the probe will determine what orientation of magnetic field will be detected.
The voltage generated Vh can be related to the external magnetic field by the following equation.
Where:
Vh is the voltage generated.
I is the applied direct current.
B is the component of the magnetic field that is at a right angle to the direct current in the Hall element.
Rh is the Hall Coefficient of the Hall element.
b is the thickness of the Hall element.
Probes are available with either tangential (transverse) or axial sensing elements
. Probes can be purchased in a wide variety of sizes and configurations and with different measurement ranges. The probe is placed in the magnetic field such that the magnetic lines of force intersect the major dimensions of the sensing element at a right angle. Placement and orientation of the probe is very important.
Measuring magnetic fields is a crucial aspect of various scientific and technological fields, including physics, engineering, geology, and medicine. Magnetic fields are generated by electric currents and permanent magnets and play a fundamental role in many natural phenomena. To accurately measure magnetic fields, several techniques and instruments are employed, each with its own advantages and limitations.
One commonly used method for measuring magnetic fields is the Hall effect. The Hall effect is based on the principle that when a conductor carrying an electric current is placed in a magnetic field perpendicular to the current flow, a voltage difference is induced across the conductor. This voltage difference, known as the Hall voltage, is directly proportional to the strength of the magnetic field. By measuring the Hall voltage, the magnetic field can be determined. The Hall effect is widely utilized in various applications, such as magnetometry, current sensing, and semiconductor characterization.
Another technique for measuring magnetic fields is based on Faraday’s law of electromagnetic induction. According to this law, a changing magnetic field induces an electromotive force (EMF) in a conducting loop or coil. By measuring this induced EMF using appropriate instrumentation like fluxgate magnetometers or induction coils, the strength and direction of the magnetic field can be determined. Fluxgate magnetometers are particularly sensitive instruments that utilize a ferromagnetic core to enhance their sensitivity. They find applications in areas such as geophysics, space exploration, and materials testing.
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