Some Basic Concepts and Units

Abstract

Already in 1820, Ampère discovered that a magnetic field is produced by an electrical charge in motion. He showed that the magnetic field depends on the shape of the circuit and arrived at the result

$$I=\oint{\vec{H}\cdot d\vec{l}},$$

(8.1)

which means that the current I in the conductor equals the line integral of H around an infinitely long rectilinear conductor. Performing the integration along a closed path around the conductor at a distance r leads to

$$\oint{\vec{H}\cdot d\vec{l}}=2\pi rH=1$$

(8.2)

or

$$H\frac{1}{2\pi r}.$$

(8.3)