Magnetism units are the standardized metrics used to quantify magnetic field strength, magnetic flux, and flux density in electrical systems. When you are winding a custom inductor, selecting a Hall-effect sensor, or troubleshooting a saturated transformer core, mixing up these metrics changes your core material selection and sensor trip points, often resulting in thermal runaway or a dead short in power supplies. Most commonly, builders confuse total magnetic flux with flux density, leading to catastrophic core saturation because they sized the core for the wrong physical property.
The Core Magnetism Units Reference Table
Before we calculate anything, you need a reliable translation layer between the modern SI system and the legacy CGS system. Many magnetics manufacturers (like TDK and Ferroxcube) still publish B-H curves and older application notes using CGS units. Here is the definitive conversion and specification table for the workshop.
| Unit Name | Symbol | System | Measures | Conversion / Real-World Benchmark |
|---|---|---|---|---|
| Tesla | T | SI | Flux Density | 1 T = 10,000 Gauss (MRI machine: 1.5T - 3T) |
| Gauss | G | CGS | Flux Density | 1 G = 0.0001 T (Earth's field: ~0.5 G) |
| Weber | Wb | SI | Total Flux | 1 Wb = 10^8 Maxwells (Total field lines through a loop) |
| Maxwell | Mx | CGS | Total Flux | 1 Mx = 10^-8 Wb (Common in legacy relay specs) |
| Ampere-turns/meter | A/m | SI | Field Strength | Magnetizing force applied by a coil |
| Oersted | Oe | CGS | Field Strength | 1 Oe ≈ 79.577 A/m (Coercivity of hard magnets) |
According to the National Institute of Standards and Technology (NIST), the Tesla and Weber are the strict SI standards, but the persistence of Gauss in American manufacturing means you must memorize the 10,000:1 ratio between Tesla and Gauss to survive reading legacy schematics.
Flux vs. Flux Density vs. Field Strength
The most frequent point of failure for DIY power supply builders is confusing what is being measured. Here is exactly what people commonly confuse these three concepts with, and how to separate them.
- Magnetic Flux (Weber / Maxwell): The total number of magnetic field lines passing through a given area.
- Magnetic Flux Density (Tesla / Gauss): The concentration of those field lines per square meter (or square centimeter). This is what causes core saturation.
- Magnetic Field Strength (A/m / Oersted): The external magnetizing force applied by your wire coil, independent of the core material inside it.
Worked Example: Sizing a Transformer Core to Avoid Saturation
Let us apply these magnetism units to a real-world scenario: checking a ferrite core for saturation in a forward converter transformer. If the flux density exceeds the material's limit, the core's permeability drops to that of air, inductance collapses, and your primary switch explodes.
The Setup:
- Core: EE-25 ferrite core (Standard MnZn material, e.g., Ferroxcube 3C90).
- Effective Cross-Sectional Area ($A_e$): $40 \text{ mm}^2$, which is $0.00004 \text{ m}^2$.
- Saturation Flux Density ($B_{sat}$): The datasheet specifies $0.32 \text{ T}$ (or 3200 Gauss) at an operating temperature of 100°C.
The Calculation:
We need to find the maximum total magnetic flux ($\Phi_{max}$) the core can handle before it saturates. The formula linking flux density ($B$) and total flux ($\Phi$) is:
$$\Phi = B \times A$$
Plugging in our real values:
$$\Phi_{max} = 0.32 \text{ T} \times 0.00004 \text{ m}^2$$
$$\Phi_{max} = 0.0000128 \text{ Wb} \text{ (or } 12.8 \text{ } \mu\text{Wb)}$$
By keeping your total flux under $12.8 \text{ } \mu\text{Wb}$, you ensure the flux density stays below the $0.32 \text{ T}$ threshold. For a deeper dive into how these values plot on a hysteresis loop, All About Circuits provides excellent visual breakdowns of B-H curve mapping.
Where You Meet Magnetism Units in Practice
You will not just see these units in textbooks; they dictate component selection on the bench every day. Here is where these specific magnetism units dictate your hardware choices.
1. Hall-Effect Sensor Trip Points (Gauss)
When building a brushless DC motor controller or a limit switch, you will use Hall-effect sensors like the ubiquitous Allegro A3144. The datasheet for the A3144 lists its operate point ($B_{OP}$) at roughly 30 to 50 Gauss (0.003 to 0.005 T). If you try to trigger this sensor with a weak ceramic magnet that only outputs 15 Gauss at the sensor's surface distance, your circuit will never register the pulse. You must upgrade to an N42 or N52 neodymium magnet to guarantee a clean logic transition.
2. Neodymium Magnet Grading (Tesla)
When sourcing magnets for a generator or magnetic brake, the "N" rating (e.g., N52) refers to the Maximum Energy Product in Mega-Gauss-Oersteds (MGOe), but the surface field strength is what you actually measure. An N52 magnet typically yields a surface flux density of 1.4 to 1.48 Tesla (14,000 to 14,800 Gauss). If your design requires a 1.2 T air gap, you must account for the inverse-cube drop-off of magnetic field strength over distance.
3. Transformer B-H Curves (A/m vs. Tesla)
When reading a TDK or Magnetics Inc. datasheet for a toroidal core, the X-axis of the B-H curve is plotted in A/m (or Oersteds), and the Y-axis is Tesla (or Gauss). The "knee" of the curve tells you exactly how much magnetizing force (Ampere-turns) you can apply before the flux density (Tesla) flatlines. Pushing past the knee means you are just generating heat in the copper windings with zero increase in magnetic transfer.
FAQ: Troubleshooting Magnetism Measurements
Q: Can I measure Tesla or Gauss accurately with my smartphone?
A: Only for rough, low-strength fields. Smartphone magnetometers (using the I2C interface to apps like Physics Toolbox) are generally accurate up to about 100 mT (1000 Gauss). However, they saturate internally near strong neodymium magnets and lack the directional transverse/axial probes required for mapping motor stators. For bench work, invest in a dedicated Gauss meter like the AlphaLab GM-2 or a Honeywell SS49E linear sensor wired to a multimeter.
Q: Why do some inductor datasheets specify current limits in Ampere-turns instead of just Amps?
A: Because Ampere-turns (A/m) measures the actual magnetic field strength applied to the core. A coil with 10 turns carrying 1 Amp produces the exact same magnetizing force (10 Ampere-turns) as a coil with 5 turns carrying 2 Amps. The core does not care about your current; it only cares about the total Ampere-turns trying to align its magnetic domains.
Q: What is the difference between Remanence ($B_r$) and Coercivity ($H_c$)?
A: Remanence is the flux density (measured in Tesla or Gauss) left in a material after you remove the external field—this is what makes a permanent magnet "permanent." Coercivity is the reverse magnetic field strength (measured in A/m or Oersteds) required to force that remanence back down to zero. Hard magnets (like N52) have high coercivity; soft transformer cores (like 3C90 ferrite) have extremely low coercivity so they can switch directions 100,000 times a second without overheating.
