Wind a wire into a long coil and the field inside comes out uniform: $B = \dfrac{\mu_0 N I}{L}$, read aloud B equals mu-nought N I over L. $B$ is the field inside in teslas; $N$ is the total number of turns, a bare count with no units; $I$ is the current in amperes; and $L$ is the length of the coil in metres — the coil's own length, NOT the length of wire wound onto it. Look at what that means: $N/L$ is turns per metre, so the field only cares how tightly the turns are packed, never how many there are in total.

Now count how much of a field gets through a loop. That is magnetic flux: $\Phi = BA\cos\theta$, read aloud phi equals B A cosine theta. $\Phi$ (the Greek letter phi) is the flux in webers (Wb); $B$ is the field in teslas; $A$ is the loop's area in square metres; and $\theta$ is the angle between the field and the loop's NORMAL — the imaginary spike sticking straight out of the loop's face. Not the loop's plane. Face-on to the field: $\theta = 0$, cosine is 1, maximum flux. Edge-on: $\theta = 90^\circ$, cosine is 0, and nothing gets through at all.

One weber is exactly one tesla-square-metre — flux earns its own unit because it is the quantity the next lesson differentiates. And watch the two conversions this page keeps setting: coil lengths arrive in centimetres, loop areas in square centimetres, and a square metre holds ten thousand square centimetres, not a hundred.