The SI units system is the most widely used measurement system, while CGS units are used mainly in the US. CGS system is still relevant all around the world to describe mainly magnetic quantities, thanks to its simple comparisons of B and H; and better scaling of units. Therefore, it complicates communication within the community dealing with magnetic quantities.
Except for different unit systems (SI & CGS), another important aspect, is to understand the difference between magnetic field strength H and magnetic field induction B which are often misunderstood. Very simply, H can be understood as the effort you put into magnetic field generation, and B is the outcome result caused by field H, which can be stronger depending on what is affected by the magnetic field. Their difference is given by multiplication factor µ, called magnetic permeability and \( B = µH \). Magnetic permeability is further given by \( µ = µ_0 . µ_r \), where \( µ_0 \) is the permeability of free space (vacuum) and \( µ_r \) is the relative permeability of the affected material (its value is equal to 1 for the vacuum).
| Physical quantity | Notation | CGS system | SI system |
|---|---|---|---|
| Vacuum permeability | \( µ_0 \) | 1 (dimensionless) | \( 4.π.10^{-7} \) H/m (Henry/meter) |
| Magnetic field strength | H | 1 Oe (Oersted) | \( \frac{10^{3}}{4.π} \) A/m ≈ 80 A/m (Ampere/meter) |
| Magnetic induction. | B | 1 G (Gauss) | \( 10^{-4} \) T (Tesla) |
Since the vacuum permeability is dimensionless and has a value of 1 in the CGS system, there is no difference between the values of magnetic field strength and induction e.i. H = 325 Oe and B = 325 G. On the other hand, in the SI system, H = 325 A/m is equal by strength to B = 408 µT. Fortunately, there is a quick calculation between CGS and SI units, where 408 µT = 0.000408 T = 4.08 G (shift the decimal point by 4 places). Also, an approximative rule is that 1 Oe is roughly 80 A/m.
\[ H_{SI} = {\frac{10^{3}}{4.π}}H_{CGS} \]
\[ H_{CGS} = {\frac{4.π}{10^{3}}}H_{SI} \]
\[ B_{SI}=10^{-4}B_{CGS} \]
\[ B_{CGS}=10^{4}B_{SI} \]
\[ µ_{SI} = 4π10^{-7}µ_{CGS} \]
\[ µ_{CGS} = {\frac{1}{4π10^{-7}}}µ_{SI} \]