A magnetic field unit quantifies the strength and direction of magnetic influence at a specific point in space, dictating how strongly it will push on moving charges or magnetize materials. On the bench, getting these units right dictates whether your transformer core saturates, which Hall effect sensor you must buy, and how thick your mu-metal EMI shielding needs to be. The most common trap for hobbyists and junior engineers is confusing magnetic flux density (the B-field, measured in Tesla or Gauss) with magnetic field strength (the H-field, measured in Amperes per meter or Oersted). Mix these up when ordering sensors or designing inductors, and your circuit will fail in ways your multimeter won't easily explain.
The Core Definition and the B-Field vs. H-Field Divide
To make the right component choices, you have to separate the cause of the magnetic field from the effect. This is where the B-field and H-field split matters.
Think of it like traffic on a highway. The H-field (Magnetic Field Strength) is the number of cars entering the on-ramp—it's the external driving force, generated purely by the current flowing through your wire coil, regardless of what the road is made of. The B-field (Magnetic Flux Density) is the actual traffic density on the highway. If the highway is built with high-capacity lanes (a high-permeability core material like iron), the traffic density (B-field) multiplies massively compared to an empty dirt road (air).
The Four Magnetic Field Units That Matter
While the SI system has standardized around the Tesla, the CGS (centimeter-gram-second) system's Gauss and Oersted still dominate American manufacturing datasheets and legacy motor specifications. Here is the exact translation matrix you need at your desk.
| Unit Name | Symbol | Measures | System | Exact Conversion |
|---|---|---|---|---|
| Tesla | T | B-field (Flux Density) | SI | 1 T = 10,000 Gauss |
| Gauss | G | B-field (Flux Density) | CGS | 1 G = 0.1 milliTesla (mT) |
| Ampere/meter | A/m | H-field (Field Strength) | SI | 1 A/m ≈ 0.01256 Oersted |
| Oersted | Oe | H-field (Field Strength) | CGS | 1 Oe ≈ 79.577 A/m |
For practical bench work, you will almost exclusively use milliTesla (mT) for sensor selection and Gauss (G) when reading magnet supplier specs. According to the NIST SI guidelines, Tesla is the official standard, but legacy engineering texts still heavily rely on Gauss for permanent magnet characterization.
Worked Example: Sizing a Hall Sensor for a BLDC Motor
Let's look at a real-world failure mode caused by ignoring unit conversions. You are building a custom BLDC motor controller and need to measure the rotor's magnetic field to trigger commutation. You grab a surface neodymium magnet and a supplier spec sheet says the surface field is 400 Gauss.
You select the popular Texas Instruments DRV5055 Hall effect sensor. Specifically, you pick the DRV5055A1 variant, which has a sensitivity of 50 mV/mT. You power it with a 3.3V rail, meaning its quiescent (zero-field) output is 1.65V, and its maximum linear output is roughly 3.3V.
Step 1: Convert Gauss to milliTesla.
400 Gauss × 0.1 = 40 mT.
Step 2: Calculate the output voltage delta.
40 mT × 50 mV/mT = 2000 mV (2.0V).
Step 3: Add to quiescent voltage.
1.65V + 2.0V = 3.65V.
Where You Meet This in Practice
You will run into magnetic field unit limits in three specific areas of electrical and electronics work:
- Transformer and Inductor Core Saturation: Standard silicon steel transformer cores saturate at roughly 1.5 T to 2.0 T (15,000 to 20,000 Gauss). If your H-field (driven by primary current) pushes the B-field past this limit, the core's permeability drops to that of air, inductance collapses, and your primary winding acts like a dead short, blowing your MOSFETs.
- EMI Shielding with Mu-Metal: Mu-metal is incredible at redirecting low-frequency magnetic interference, but it has a very low saturation point of about 0.8 T (8,000 Gauss). If you place it too close to a strong neodymium magnet or a high-current busbar, it saturates and becomes transparent to the EMI. You must calculate the B-field at the shield boundary to ensure it stays under 0.8 T.
- Magnetic Encoder Resolution: When using absolute magnetic encoders (like the AS5600) for robotics joints, the datasheet will demand a specific B-field window at the die surface—typically 30 mT to 70 mT (300 to 700 Gauss). Too weak, and the ADC noise floor ruins your angular resolution; too strong, and the internal AGC (Automatic Gain Control) maxes out.
Decision Tree: Picking the Right Unit and Sensor
Stop guessing which sensor to buy. Use this decision matrix to lock in your measurement unit and select a concrete part number based on your physical application.
| IF your application is... | THEN measure in... | AND buy this specific sensor IC |
|---|---|---|
| High-field motor commutation or current sensing (>100 mT) | milliTesla (mT) | Allegro A1324 (High sensitivity, ratiometric, handles strong fields without clipping) |
| Low-field absolute angle encoding or joystick (<50 mT) | microTesla (µT) or Gauss | Melexis MLX90393 (3-axis, I2C/SPI, ultra-low noise floor for weak diametrical magnets) |
| AC mains leakage or geomagnetic mapping (Fluctuating µT) | microTesla (µT) or milliGauss | Honeywell HMC5883L (or modern equivalent like BMM150) for multi-axis fluxgate/magnetometer mapping |
| Proximity switching (Binary presence of a magnet) | Gauss (Threshold only) | TI DRV5032 (Ultra-low power digital switch, ignores exact field strength, just trips at ~3 mT) |
Frequently Asked Questions
Can I use a standard multimeter to measure magnetic field units?
No. Standard multimeters measure electrical properties (voltage, current, resistance). To measure Tesla or Gauss directly, you need a dedicated Gaussmeter (teslameter) with a Hall probe, or you must build a circuit using a Hall effect IC and read the analog voltage output with your meter.
Why do magnet suppliers use Gauss while sensor datasheets use Tesla?
It is a historical hangover. The permanent magnet manufacturing industry in the US and Asia standardized on the CGS system (Gauss) decades ago, while the semiconductor industry, which designs Hall sensors, operates strictly in SI units (Tesla/milliTesla). You must manually bridge this gap during component selection.
What is the default recommendation if I am unsure of the field strength?
Default to measuring in milliTesla (mT) and select a ratiometric linear Hall sensor with a medium sensitivity (e.g., 25 mV/mT). This provides the widest safe dynamic range for standard 3.3V and 5V microcontroller ADCs without risking signal clipping from unexpectedly strong stray fields.
