# What unit system is used in Feynman's Lectures on Physics, Pt. 2

In part II, lecture 18, of Feynman's Lectures on Physics, on Table 18-1 Feynman writes Ampère's law as $$c^2 \nabla \times \vec{B} = \frac{\vec{j}}{\epsilon_0} + \frac{\partial \vec{E}}{\partial t}.$$

What unit system is this in? It's not obviously Gaussian, which states, $$c \nabla \times \vec{B} = 4 \pi \vec{j} + \frac{\partial \vec{E}}{\partial t}.$$

Nor does it appear to be formulated in SI, which states,

$$\frac{1}{\mu_0} \nabla \times \vec{B} = \vec{j} + \epsilon_0 \frac{\partial \vec{E}}{\partial t}.$$

• This appears to be using SI convention, taking advantage of the relation $\epsilon_0\mu_0 c^2 = 1$ – By Symmetry Oct 21 at 17:49
• @BySymmetry Shouldn't that be an answer instead of a comment? – Emilio Pisanty Oct 21 at 18:03

Multiply $$c^2 \nabla \times \vec{B} = \frac{\vec{j}}{\epsilon_0} + \frac{\partial \vec{E}}{\partial t}$$ through by $$\epsilon_0$$, and substitute in the identity $$c^2 = \frac1{\epsilon_0\mu_0} ,$$ and it will be quickly revealed to be in the SI form you quote.