Standard metal detectors, commonly seen at airports and public venues, identify objects that interact with a generated electromagnetic field. An item’s detectability depends not just on whether it is metal, but on how its physical properties respond to the specific technology used. Understanding this underlying science explains why some metallic objects may fail to trigger an alarm while others, much smaller, immediately cause an alert.
The Physics of Metal Detection
Standard security detectors operate on the principle of electromagnetic induction, using a controlled magnetic field to search for conductive materials. A transmitting coil within the gate generates a primary, alternating magnetic field that extends into the walk-through area. This continuous or pulsed field acts as the detector’s probe, constantly looking for a disturbance.
When a conductive object passes through this field, the changing magnetism induces tiny electrical currents within the object, known as eddy currents. These eddy currents immediately generate their own secondary magnetic field, which radiates outward. Receiving coils within the gate are tuned to sense this secondary field.
The detector’s electronics analyze the strength and phase of the detected secondary field. If the signal exceeds a predefined threshold, the device triggers an alarm, signaling the presence of a conductive object. Detectors use various technologies, such as Very Low Frequency (VLF) systems or Pulse Induction (PI) technology, which sends out short bursts of energy before listening for the magnetic response.
Material Properties Determining Detectability
The strength of the generated secondary magnetic field depends on two primary material characteristics: electrical conductivity and magnetic permeability. Electrical conductivity dictates how easily the object can sustain the induced eddy currents. Metals with high conductivity, such as copper and aluminum, create a pronounced secondary field, making them highly detectable.
Magnetic permeability refers to a material’s ability to support the formation of a magnetic field when exposed to an external one. Ferrous metals, like iron and many steels, have high magnetic permeability, meaning they are easily magnetized and create a distinct magnetic response. Non-ferrous metals that are poor conductors are difficult to detect because they fail to produce both a strong magnetic response and robust eddy currents.
A metal’s response is a combination of these two factors, which the detector’s electronics analyze to classify the material type. Highly conductive, non-ferrous materials generate a signal primarily from eddy currents. Conversely, highly permeable, ferrous materials generate a signal dominated by their magnetic properties. Materials with low conductivity and permeability produce a minimal signal, presenting the greatest challenge to detection.
Materials That Bypass Standard Detectors
Materials most likely to bypass standard security screening are those engineered for minimal interaction with electromagnetic fields. Pure titanium, frequently used in medical implants, has notably low electrical conductivity compared to metals like gold or silver. Its non-ferromagnetic nature means it lacks the high magnetic permeability that would make it easily detectable.
Certain types of stainless steel, specifically austenitic grades like 304 and 316, also present a challenge. These alloys contain high levels of chromium and nickel, rendering them non-magnetic and minimizing the magnetic response. Combined with low electrical conductivity, small objects made of these stainless steels often generate an insufficient signal to trigger an alarm.
Non-conductive materials used as metal substitutes can pass through unimpeded. High-density engineering plastics, ceramics, and carbon fiber composites are entirely non-metallic and lack the electrical conductivity necessary to form eddy currents. These materials are increasingly used in specialized tools because they offer high strength without an electromagnetic signature. Cobalt-chrome-molybdenum alloys, common in biomedical applications, also exhibit similar low magnetic and conductive properties.
Practical Factors Influencing Detection Reliability
Even for theoretically detectable materials, the reliability of an alarm is influenced by external and operational factors. The most significant variable is the sensitivity setting of the detector, which security personnel adjust based on the required security level. A detector set to low sensitivity may ignore objects that would alarm a machine set to its highest calibration.
The physical characteristics of the object are also important, including its size, shape, and orientation as it passes through the field. For example, a large, flat sheet of low-conductivity metal may be detected more easily than a tiny, spherical piece of highly conductive metal. Wire-like objects are challenging because their signal varies dramatically depending on their orientation relative to the detector’s coils.
The size of the detector’s aperture, the opening an individual walks through, also affects the field strength. The magnetic field is weakest at the geometric center of the opening. This means an object passing directly through the middle is less likely to be detected than one traveling closer to the side panels. Detection is always a probabilistic event, balancing the material’s inherent properties with the machine’s setup and the object’s presentation.