Thesis

[matches/honours.git] / thesis / appendices / electron_gun_circuit.tex~

diff --git a/thesis/appendices/electron_gun_circuit.tex~ b/thesis/appendices/electron_gun_circuit.tex~
index 399270c..c640556 100644 (file)
--- a/thesis/appendices/electron_gun_circuit.tex~
+++ b/thesis/appendices/electron_gun_circuit.tex~
@@ -1,5 +1,7 @@
 
-\chapter*{Appendix - Electron Gun Control Circuit}
+\section{Electron Gun Control Circuit}
+
+\subsection{Control Circuit}
 
 The control circuit diagram for the electron gun is shown in Figure \ref{electron_gun.pdf}. The wiring of the circuit, including resistors and potentiometers, was incoroprated into a single box, with external connections available for the power supplies, ammeters, electron gun, and sample holder. Both the components and operation of this circuit are straightforward; we will give a brief overview here for completeness.
 
@@ -40,8 +42,18 @@ The control circuit diagram for the electron gun is shown in Figure \ref{electro
 \end{itemize}
 \begin{landscape}
 \begin{center}
-       \includegraphics[scale=0.80]{figures/egun/electron_gun.pdf}
+       \includegraphics[scale=0.75]{figures/egun/electron_gun.pdf}
        \captionof{figure}{Circuit Diagram for Electron Gun Control}
        \label{electron_gun.pdf}
 \end{center}
+
 \end{landscape}
+
+\section{The Ammeters}
+
+An ideal ammeter has no input resistance. In reality, it is not the current that is measured, but the voltage accross a fixed input resistor. This voltage can either be amplified, or the resistance increased, for measuring a smaller current.
+
+Since there is a voltage drop across the ammeter, the potential of the surface relative to the cathode is actually $U + I R$, where $R$ is the input resistance of the ammeter.
+
+The 602 and 610B electrometers both provide a large range of scales and amplifier settings for current measurement. Using a low scale setting increases the input impedance, which increases the potential drop accross the ammeter. However, using a large amplifier gain increases noise; hence there is a trade off. For the 602 and 610B electrometers, a significant drift (typical +5\% of scale in 10min) in the zero level was also observed at high amplifier gains, whilst low gains appeared more stable (+10\% noted after 2 days).
+