
Why is the tangent not really horizontal?
\documentclass{article}
\usepackage{tikz}
\usepackage{circuitikz}
\usepackage{pst-solides3d}
\usetikzlibrary{shapes,decorations}
\usetikzlibrary{positioning,arrows}
\tikzset{
state/.style={circle,draw,minimum size=6ex},
arrow/.style={-latex, shorten >=1ex, shorten <=1ex}}
\begin{document}
\begin{tikzpicture}
\node[name=D,draw,circle,inner sep=3pt,fill=gray!50] at (17,.5) {};
%d
\draw
(D.north) to (16.65,.65)
(D.south) to (16.65,.35)
;
\draw[latex-]
(16.75,.65) to (16.75,1);
\draw[latex-]
(16.75,.35) to (16.75,0);
\node at (16.45,.5) {$d$};
\end{tikzpicture}
\end{document}

D.northis on the very outside of the circle, so the middle of your incident line is aligned with the outside of the circle line. There are a couple of ways to solve this depending on how technical you are prepared to be. – Andrew Stacey Sep 07 '14 at 22:32outer sep=0mmwould solve it. – percusse Sep 07 '14 at 22:35\documentclass[tikz]{standalone} \usepackage{circuitikz} \usetikzlibrary{positioning} \tikzset{ state/.style={circle,draw,minimum size=6ex}, arrow/.style={-latex, shorten >=1ex, shorten <=1ex}} \begin{document} \begin{tikzpicture} \node[name=D,draw,circle,inner sep=3pt,fill=gray!50] at (17,.5) {}; %d \draw (D.north) -- +(-.35,0) (D.south) -- +(-.35,0) ; \draw[latex-] (16.75,.65) to (16.75,1); \draw[latex-] (16.75,.35) to (16.75,0); \node at (16.45,.5) {$d$}; \end{tikzpicture} \end{document}– cfr Sep 07 '14 at 22:37