Reference Directions#

We can illustrate the currents flowing through a circuit and the voltages across various points inside it using the so called reference arrows. We are free to arbitrarily choose their direction, so long as we stick to our choice once we have made it. This is because the actual direction is determined by the combination of algebraic sign and reference direction:
- A positive current means that the direction in which conventional current flows is the same as the chosen reference direction. A negative current means that the direction in which conventional current flows is the opposite to the chosen reference direction.
- A positive voltage means that the reference arrow points from the point with a higher potential towards the point with a lower potential. A negative voltage means that the reference arrow points from the point with a lower potential towards the point with a higher potential.

Physically, these reference arrows correspond to the way currents and voltages are measured. Ammeters are calibrated to output positive numbers when the flow of conventional current is from their positive terminal to their negative terminal and to output negative numbers when conventional current flows in the opposite direction. Similarly, voltmeters are calibrated to output positive numbers when the electric potential is higher at their positive terminal and to output negative numbers when the electric potential is higher at their negative terminal .