The Direct Answer: Units and Tools for Magnetic Field Measurement
Magnetic field is measured in Tesla (T) in the SI system and Gauss (G) in the older CGS system. For practical bench and field work, the conversion is absolute: 1 Tesla = 10,000 Gauss. Because a Tesla is a massive unit, hobbyists and technicians almost exclusively use Gauss (or milliTesla, where 1 mT = 10 G) for component-level testing.
It is critical to distinguish between the two distinct vector quantities often lumped together as "magnetic field":
- Magnetic Flux Density (B): Measured in Tesla or Gauss. This is what a Hall effect sensor actually reads. It represents the concentration of magnetic field lines passing through a given area.
- Magnetic Field Strength (H): Measured in Amperes per meter (A/m) or Oersteds (Oe). This represents the magnetizing force generated by a current-carrying coil, independent of the core material.
According to the NIST Guide to the SI, Tesla is the standard derived unit for magnetic flux density (kg·s⁻²·A⁻¹). When you buy a handheld meter, you are buying a B-field meter (a gaussmeter or teslameter), not an H-field meter.
Meter Setup and Probe Placement for Hall Effect Testing
Measuring magnetic flux density accurately requires understanding your probe geometry. Hall effect sensors are directional. If you use the wrong probe orientation, your readings will be useless due to cosine error.
Meter Setup Block: Standard Bench Gaussmeter
- Dial/Mode: Set to DC (for permanent magnets, DC solenoids) or AC (for transformers, AC motors). If measuring a pulsing DC field (like a brushed motor commutator), use the AC mode with a low-pass filter if your meter supports it.
- Lead Jacks: Connect the Hall probe to the dedicated high-impedance input. Do not use standard multimeter current jacks; Hall probes require a constant current excitation source provided only by the gaussmeter's specific port.
- Range: Start on the highest range (e.g., 20,000 G or 2 T) to prevent sensor saturation. A strong N52 neodymium magnet will instantly peg a 200 mT (2,000 G) scale and may temporarily blind the sensor.
Probe Placement: Axial vs. Transverse
Your probe placement dictates the accuracy of your measurement. Gaussmeters come with two primary probe types:
- Transverse Probes: The Hall element is mounted flat near the tip. The sensitive axis is perpendicular to the probe shaft. Use this for measuring the surface field of a flat magnet or the air gap between two poles. Place the flat face of the probe flush against the target surface.
- Axial Probes: The Hall element is mounted in-line with the tip. The sensitive axis is parallel to the probe shaft. Use this for measuring the field inside a solenoid, a pipe, or a deep bore. Insert the tip directly into the field path.
When measuring magnetic leakage around mains-fed transformers, busbars, or industrial motor stators, you are working in a high-voltage environment. While the magnetic probe itself is low-voltage and isolated, your body and meter are not. If you are using a clamp meter to indirectly measure the magnetic field via current, the meter must be rated CAT III 600V or CAT IV 600V. Never let a fragile glass-epoxy Hall probe touch bare, energized mains conductors; the probe dielectric is not rated for line voltage and will arc over, destroying the sensor and posing a lethal shock hazard.
Expected Readings: Good vs. Bad Magnetic Flux Densities
Knowing what the meter should display is the difference between debugging a circuit and chasing ghosts. Below are the baseline expected readings for common bench and jobsite components. These values assume a standard ambient temperature of 25°C (77°F).
| Test Point / Component | Probe Type | Good Reading (Nominal) | Bad Reading (Failure/Degradation) |
|---|---|---|---|
| N52 Neodymium Magnet (Surface) | Transverse (Flush) | 12,500 – 14,200 G (1.25 – 1.42 T) | < 10,000 G (Indicates thermal demagnetization or physical cracking) |
| Ferrite/Ceramic Magnet (Surface) | Transverse (Flush) | 2,500 – 4,000 G (0.25 – 0.40 T) | < 2,000 G (Severe aging or reverse-field demagnetization) |
| Earth’s Magnetic Field (Outdoor) | Tri-axial or Omni | 0.25 – 0.65 G (25 – 65 µT) | > 2.0 G (Indicates local ferrous interference or meter zero-error) |
| 12V DC Solenoid (Air Gap, 500 turns) | Axial (Centered) | 80 – 150 G (Depends on core geometry) | < 30 G (Indicates shorted turns or high-resistance coil connection) |
| Mains Transformer (Leakage Flux at case) | Transverse (AC Mode) | < 5 G AC (Well-shielded, low leakage) | > 50 G AC (Poor lamination stacking, loose core bolts, or saturation) |
Common Mistakes That Give Misleading Gauss Readings
If your readings do not match the expected values above, do not immediately assume the component is faulty. Hall effect sensors are highly susceptible to environmental and mechanical errors. According to Texas Instruments' Hall Effect Sensor design guidelines, the following error modes are the most common on the bench:
- Cosine Error (Angular Misalignment): Hall sensors only measure the vector component of the field perpendicular to the die. If your transverse probe is tilted just 15 degrees off the magnet's surface, your reading will drop by nearly 4% (cos 15° = 0.965). At 45 degrees, you lose 30% of your reading. Always use a non-magnetic jig or fixture to hold the probe perfectly flat.
- Temperature Drift: The sensitivity of a silicon Hall element changes with temperature. Standard uncompensated sensors drift by roughly -0.1% to -0.3% per °C. If you measure a motor stator immediately after a 30-minute run (where surface temps hit 60°C), your reading will artificially drop by 10% or more compared to a cold bench test. Always let the DUT cool to 25°C for baseline mapping.
- Zero-Offset Shift: Dropping a Hall probe or exposing it to a massive magnetic spike can shift the sensor's zero point. Before every testing session, move the probe at least 3 feet away from any ferrous metals or magnets, press the "Zero/Null" button on your meter, and verify it reads 0.00 G ± 0.1 G.
- Measuring AC with a DC Meter: A standard DC gaussmeter will read zero (or a fluctuating, meaningless noise floor) when placed against an AC electromagnet or a running AC motor. You must switch the meter to AC mode, which engages an internal RMS-to-DC converter tuned for 50/60 Hz.
When measuring the surface field of a bare neodymium magnet, the probe's fragile epoxy tip can easily scratch or chip. Place a single sheet of standard 20lb printer paper (approx. 0.1 mm thick) between the magnet and the transverse probe. The field strength drops by less than 1% over 0.1 mm at the surface of a large magnet, but it saves your $150 probe from catastrophic physical damage.
Decision Tree: Which Sensor or Meter Should You Buy?
Choosing the right tool depends entirely on the field type (AC vs DC), the required accuracy, and whether you are measuring a standalone component or embedding a sensor into a PCB. Follow this decision path to select your equipment:
- IF you need to measure static DC fields (permanent magnets, DC motor stators, magnetic latches) on the bench with ±1% accuracy...
→ Look for a dedicated handheld DC Gaussmeter with a transverse probe. - IF you need to measure AC magnetic leakage (transformers, mains inductors, 50/60 Hz power cables)...
→ Look for an AC/DC Gaussmeter with true-RMS AC capability, or use a CAT III leakage clamp meter to measure the electrical current generating the field. - IF you are designing a custom PCB to detect magnetic proximity or measure fields inside a closed enclosure...
→ Look for an analog-output linear Hall effect IC (e.g., TI DRV5055 or Allegro A1324). - IF you are doing university-level physics research requiring 0.01% accuracy and temperature compensation...
→ Look for a laboratory-grade Teslameter (e.g., Lake Shore 425) with an active temperature-compensated probe.
The Default Recommendation
For 90% of electrical hobbyists, DIYers, and bench technicians who need to know what a magnetic field is measured in and actually want to see the number, the concrete pick is the AlphaLab GM-2 Gaussmeter.
Priced around $350, the GM-2 includes both an axial and a transverse probe, measures DC fields up to 20,000 Gauss (2 Tesla), and features a simple, rugged interface that survives the workbench. It provides the exact numerical feedback needed to verify magnet grades, check solenoid air gaps, and map motor stators without the $3,000 price tag of laboratory-grade Lake Shore equipment. If your work strictly involves AC mains leakage, skip the gaussmeter entirely and buy a Fluke 368 FC leakage current clamp to measure the source current safely.
