File:DefMagField.svg

\documentclass[crop,border=5,tikz,convert={outext=.svg,command=\unexpanded{pdf2svg
    \infile\space\outfile}},multi=false]{standalone}
\usetikzlibrary{math,calc,through,shapes,patterns,decorations.pathmorphing}

\begin{document}
\begin{tikzpicture}[>=latex,scale=1]
   \foreach \x in {1,2,...,4}{
    \foreach \y in {1,2,...,4}{
      \draw[thick,black!50!blue] (\x,\y) circle (3mm);
      \draw[thick,black!50!blue]
      (\x-0.21213,\y-0.21213)--(\x+0.21213,\y+0.21213);
      \draw[thick,black!50!blue]
      (\x+0.21213,\y-0.21213)--(\x-0.21213,\y+0.21213);
    }
  }
  \node[black!50!blue] at (0.4,2.5) {$\vec{B}$};
  \draw[ultra thick,->,black!50!green] (2.5,2.5)--+(0,-2.5)
  node[above left] {$\vec{F}_{\mathrm{m}}$};

  \draw[ultra thick,black] (1.5,5)--(1.5,2.5)--(3.5,2.5)--(3.5,5);
  \draw[ultra thick,<-,black] (1.5,4)--(1.5,3.5);
  \draw[ultra thick,->,black] (3,2.5)--(2.3,2.5);
  \node[above] at (2.5,2.5) {$I$};
  \draw[ultra thick,->,black] (3.5,4)--(3.5,3.5);
  \draw[<->,black] (1.5,4.6)--(3.5,4.6) node[midway,above]{$b$}
  node[above right]{$n$};

  \node(E)[black!50!blue] at (2.5,-0.75) {\LARGE
    $|\vec{B}|= B:=\frac{F_{\mathrm{m}}}{I\cdot b\cdot n}$};
  \node(S)[black] at (3.5,1.5) {\footnotesize $\vec{B}\perp\vec{F}_{\mathrm{m}}\perp\vec{I}$};
\end{tikzpicture}
\end{document}