A magnetic field is an area around a magnet or a moving electric charge where magnetic forces can be felt. Understanding and measuring their strength is important for countless technological applications and scientific endeavors.
Primary Units of Measurement
Magnetic field strength is primarily quantified using two main units: the Tesla and the Gauss. The Tesla (T) is the standard International System of Units (SI) measure for magnetic flux density, often denoted as the B-field. One Tesla is defined as the magnetic flux density that exerts a force of one Newton on a one-meter length of wire carrying one Ampere of current, when the wire is perpendicular to the field.
The Gauss (G) is an older unit from the centimeter-gram-second (CGS) system, also measuring magnetic flux density. While largely superseded by the Tesla in scientific and engineering contexts, the Gauss remains commonly used in some specialized fields. One Tesla is equivalent to 10,000 Gauss.
A distinct but related unit is the Ampere per meter (A/m), which measures magnetic field strength, known as the H-field. The B-field (measured in Tesla or Gauss) describes the force a magnetic field exerts within a given medium, while the H-field describes the field’s intensity regardless of the material it passes through.
Understanding Field Strength in Context
To grasp the scale of magnetic fields, considering real-world examples is helpful. The Earth’s magnetic field, which protects our planet from solar radiation, is relatively weak, ranging from approximately 25 to 65 microteslas (µT) at its surface, or about 0.25 to 0.65 Gauss. A common refrigerator magnet, for comparison, generates a field of around 5 milliteslas (mT), which is equal to 50 Gauss.
Medical imaging devices, such as Magnetic Resonance Imaging (MRI) machines, utilize significantly stronger magnetic fields for detailed internal body scans. Clinical MRI scanners typically operate with fields between 0.5 Tesla and 3.0 Tesla. Some advanced research MRI systems can reach up to 7 Tesla, while specialized instruments like Nuclear Magnetic Resonance (NMR) spectrometers can operate at 21.1 Tesla for chemical analysis.
Beyond medical applications, scientific research and industrial uses involve even more powerful magnets. Continuous magnetic fields can reach strengths of up to 45 Tesla in specialized laboratories. For fleeting moments, pulsed magnets can generate non-destructive fields of 100 Tesla, with some experimental, destructive setups achieving fields as high as 2,800 Tesla.
How Magnetic Fields Are Measured
Magnetic fields are measured using specialized instruments called magnetometers. These devices are designed to detect and quantify the strength and often the direction of a magnetic field. Magnetometers are widely used across various fields, including geophysical surveys, navigation, and material analysis.
Different types of magnetometers exist, each suited for specific applications and field strengths.
Hall Effect Sensors
Hall effect sensors, for instance, measure magnetic fields by detecting a voltage produced across a conductor when current flows through it perpendicular to a magnetic field.
Superconducting Quantum Interference Devices (SQUIDs)
Superconducting Quantum Interference Devices (SQUIDs) are highly sensitive magnetometers capable of detecting extremely weak magnetic fields, such as those generated by the human brain or heart.
Fluxgate Magnetometers
Fluxgate magnetometers are another type, often used for measuring the Earth’s magnetic field, and they operate by sensing changes in the magnetic properties of a core material.
All these instruments work by translating the interaction of the magnetic field with a sensor into a measurable electrical signal.