ahhhhhhhhhh but I have to chime in...... being an EE myself.......

by turning the knob you are changing the impedence to the circuit and thereby changing the amount of current that can flow through the plug. Ohms Law states that I = V/R when increasing the "R" you decrease "I", or current, which is what is shown on the panel meter. You are varying the current flowing through the plug by increasing or decreasing the circuit resistance, or "R". The voltage supplied to your panel is fairly constant at 12 volts (Assuming the battery is fully charged and not heavily loaded).

When you increase the impedence to the circuit you drop more voltage across the potentiometer, which is controlled by the knob, and thereby have less voltage "Felt" at the glow plug.

The voltage in the circuit as a whole is fixed at 12 volts DC. What varies is how much voltage is felt at the plug, and the current flowing through it.

As JNorton stated...... It is all related. Bottom line is that the knob adjusts how hot your plug is.......