$**$ P$26$ A long plastic rod with rectangular cross section $hw$ is wound with a coil of length $d$ consisting of $N$ closely packed turns of wire with negligible resistance. A current I runs through the mil

rigure $\frac{21\mathrm{.}1}{?}$

Using the rectangular path of length $b$ shown on the diagram in Figure $21\mathrm{.}77,$ prove that the magnetic field inside the coil is $B={}_{0}N\frac{\text{I}}{d},$ and describe the direction of vec$(B)$ inside the coil.