The measurement of magnetic strength is a fundamental practice in science and industry, used for quality control, device design, and performance verification. The term “strength” is not a single value, as a magnet’s power is quantified in several distinct ways. Different applications require measuring different properties, leading to two primary, non-interchangeable measurement categories. Understanding the specific metric being used is the first step in accurately assessing a magnet’s capabilities.
Defining Magnetic Strength and Terminology
Magnetic strength is defined by two physical concepts: the mechanical force of attraction and the intensity of the magnetic field itself. The mechanical measure is Pull Force (or Holding Force), which describes the maximum load a magnet can support before separating from a flat, ferrous surface. This value is typically measured in Newtons or Pounds.
The scientific measure is Magnetic Flux Density (B-field), which represents the total magnetic field. Its standard unit is the Tesla (T), or the Gauss (G), where one Tesla equals 10,000 Gauss.
Manufacturers often use the magnet’s Remanence (\(B_r\)), the residual flux density remaining after magnetization. The overall quality is expressed by the Maximum Energy Product (MGOe), which combines resistance to demagnetization with remanence.
Practical Measurement of Holding Force
The simplest method for determining a magnet’s strength is the pull test, which quantifies the Pull Force. This test measures the mechanical force required to detach the magnet from a standard steel fixture, making it the most practical measurement for holding or lifting applications.
The measurement uses a spring scale or digital dynamometer. The magnet is placed in full contact with a flat, thick steel plate, and the device measures the peak force registered upon separation. The pulling force must be applied perpendicular to the contact surface.
This measurement is highly dependent on the testing environment. The steel plate must be thick enough to fully saturate the magnetic circuit. The grade and surface condition of the steel fixture also influence the result, as a rough surface yields a lower pull force reading.
Technical Measurement of Field Intensity
For precise scientific or quality control purposes, the intensity of the magnetic field (B-field) is measured using specialized electronic instruments. The most common tool is the Gaussmeter or Teslameter, which provides a digital readout of the magnetic flux density at a specific point. These devices operate based on the Hall Effect principle.
The Hall Effect works by passing a constant electric current through a semiconductor strip, called a Hall probe. When the probe is placed in a magnetic field, the field exerts a force on the moving charge carriers, pushing them to one side. This charge separation creates a measurable transverse voltage (Hall voltage), which is directly proportional to the magnetic field strength.
To measure the magnet’s residual flux density (\(B_r\)), the Hall probe is typically placed in direct contact with the surface, oriented perpendicular to the field lines. Measuring the field at a distance is also common, as this value indicates the strength available for a working air gap. Another device, the Fluxmeter, measures the total magnetic flux passing through a defined area, often used to determine the overall magnetic moment of the part.
Variables Influencing Accurate Readings
Several external variables can significantly alter the accuracy and repeatability of magnetic readings.
Temperature
Temperature is a primary factor, as a magnet’s strength is inversely related to its temperature. Permanent magnets, particularly Neodymium, experience a measurable decrease in residual flux density (\(B_r\)) when heated above ambient temperatures.
Distance
Distance is a major variable in field intensity measurements. The magnetic field diminishes rapidly as the distance from the source increases. Because field strength falls off according to an inverse power law, a small error in sensor positioning can lead to a large difference in the recorded value.
Surrounding Materials
Surrounding materials introduce inaccuracies by altering the magnetic field pathways. Nearby ferrous materials can inadvertently shield or concentrate the magnetic flux, leading to skewed readings. Accurate measurements require a magnetically clean environment to ensure the recorded values represent only the field produced by the test object.