Magnetic particle testers are devices used to detect surface and near-surface defects in ferromagnetic materials. They work by applying a magnetic field to the material and then sprinkling fine magnetic particles over its surface. If there are any cracks or flaws, the particles will cluster around these areas, making them visible. This testing method is commonly used in industries such as aerospace, automotive, and manufacturing to ensure the integrity of components. It is effective for identifying issues like fatigue cracks and stress fractures, helping to prevent failures in critical applications.