Final answer:
When the analog meter's needle moves below zero, it indicates a reverse polarity condition or negative voltage, often caused by reversed lead connections to the voltage source.
Step-by-step explanation:
When the needle on an analog meter moves below zero on the scale while taking a DC voltage measurement, it indicates a reverse polarity condition or a negative voltage. This can occur if the meter's leads are connected inversely to the voltage source; the red lead should be connected to the positive terminal, and the black lead to the negative terminal. The analog meter has a galvanometer (G) that measures the current (IG), resulting in a proportional deflection of the needle due to magnetic field forces acting on the current-carrying wire. When the polarity is reversed, the needle moves in the opposite direction indicating a negative voltage.