Suggest a method for joining the connecting wires to the pencil lead at A and B.
A micrometer is used to find the mean diameter of the pencil lead. Explain how it is used.
The resistance measured, $X$, is $341\,\Omega$. The pencil lead diameter, $D$, is $0.050\,\text{cm}$. The distance $s$ between points A and B along the pencil lead is $12\,\text{cm}$. The material constant $P$ is related by $P = \frac{X\pi D^2}{4s}$. Calculate $P$ in $\Omega\,\text{cm}$. Present your answer to two significant figures.
On Fig. 2.4 opposite, plot $R/\text{k}\Omega$ on the y-axis against $l/\text{cm}$ on the x-axis. Begin the graph at the origin. Draw the straight line of best fit.
Find the gradient $G$ of the line. Clearly indicate on the graph how you found $G$.
Use your value of $P$ from (a)(iii) and $G$ from (b)(ii) to find the depth $d$ of the carbon layer in the pencil line using $d = \frac{P}{200G}\,\text{cm}$. Give the result in standard form.