Question: Here is the code.

```
\documentclass[12pt,letterpaper]{article}
\usepackage[left=20mm,top=30mm,bottom=30mm,right=20mm]{geometry}

\usepackage[siunitx]{circuitikz} % circuit package and  include electrical units in our labels

\begin{document}

\begin{center}
\begin{circuitikz} [american voltages] \draw
  (5,0) to
  (5,3) to [V, v<=15<\volt>,  a = $V_{source}$] (-1,3) --
  (-1,0) -- (1,0) to [pR, l = $Load$, a = 50k$\Omega$] (3,0) -- (5,0)

;
\end{circuitikz}
\end{center}

\end{document}

```

The resistor marked in red box is what I want to achieve.

I have tried to use `rotate` and `yscale`, but both of them don't work for me. And how can I adjust the length of the resistor?

[![Enter image description here](https://i.stack.imgur.com/r17zC.png)](https://i.stack.imgur.com/r17zC.png)

Answer: 
As supplement to @marijn answer:

[![enter image description here](https://i.stack.imgur.com/uu5u1.png)](https://i.stack.imgur.com/uu5u1.png)

```
\documentclass[12pt,margin=3mm]{standalone}
\usepackage[siunitx]{circuitikz} 
\usetikzlibrary{arrows.meta}

\begin{document}
\begin{circuitikz} [american voltages,
    > = Straight Barb
                    ] 
\ctikzset{resistors/width=1.6, resistors/zigs=7}
\draw   (0,0) to [V=15<\volt>,  a = $V_{\mathrm{source}}$] ++ (-4,0) 
        -- ++ (0,-3)
        to [pR, a = Load, l = 50<\kilo\ohm>, mirror, name=R] ++ (4,0) 
        -- (0,0)
        ;
\draw[|<->, 
      transform canvas={yshift=0.5mm}] (R.north west) -- node[fill=white, inner sep=0pt] {$V_{\mathrm{th}}$} (R.wiper);
\end{circuitikz}
\end{document}

```
The longer resistor is described in the CircuiTikZ [manual](http://mirrors.ctan.org/graphics/pgf/contrib/circuitikz/doc/circuitikzmanual.pdf) on page 52:

> 
> For the american style resistors, you can change the number of "zig-zags" by setting the key `resistors/zigs` (default value 3).
> 
> 
> 

In the manual this sentence is followed by a code example which I repeated in the MWE below.

Switching the labels and annotations can be done using `l_` instead of `l` for a label below the element and `a^` instead of `a` for an annotation above the element. Putting the arrow on the other side can be done using the `mirror` key.

Note: since you load the `siunitx` option for `circuitikz` it would make sense to use this not only for Volt but also for kOhm, which improves the spacing a bit.

MWE:

```
\documentclass[12pt,letterpaper]{article}    
\usepackage[left=20mm,top=30mm,bottom=30mm,right=20mm]{geometry}

\usepackage[siunitx]{circuitikz} % circuit package and  include electrical units in our labels

\begin{document}

\begin{center}
\begin{circuitikz} [american voltages, longpot/.style = {pR, resistors/scale=0.75,
resistors/width=1.6, resistors/zigs=6}] \draw
  (5,0) to
  (5,3) to [V, v<=15<\volt>,  a = $V_{source}$] (-1,3) --
  (-1,0) -- (1,0) to [longpot, l_ = $Load$, a^ = \qty{50}{\kohm}, mirror] (3,0) -- (5,0)

;
\end{circuitikz}
\end{center}

\end{document}

```

Result:

[![enter image description here](https://i.stack.imgur.com/0gFMz.png)](https://i.stack.imgur.com/0gFMz.png)