In the previous section it was shown, that the electrical circuit has essentially two different parts. Part of the chain, in which charges move in the direction of the action of electric forces (The DIA Fig. 16.3), called external, as part of the chain, in which charges move and the direction of action of external forces (ВDА in figure. 16.3), called internal. In other words, inner circuit is the source of electrical energy, and external — the rest of the chain.

Those points, in which an external circuit internal borders, called poles. In the external circuit, the charges move from one point to another, a potential difference; hollow, when in closed circuit the current, the potential in the external circuit from point to point decreases (in the direction from A to b on figure. 16.3). Thus, one of the poles has the greatest potential, and the other has the smallest capacity, but compared with other points in the circuit. Pole with the highest potential is called positive and indicate the sign "+", and the pole with the lowest potential is called negative and indicate the sign "—".

In diagrams of electrical circuits used symbols, shown in figure. 16.4. It is considered, that long thin feature in the designation of the source of electrical energy is a positive pole, short and fat — negative.

Diagram of a simple electrical circuit with the inclusion of measuring instruments is shown in figure. 16.5. Recall, what is the direction of the current in the external circuit accept movement of positive charges from positive pole to negative, and domestic — from negative pole to positive, although in metals the electrons move in the opposite direction.

Because in the external circuit in the direction of the current potential point-to-point fall, voltage U₁ on any site, forming part of the external circuit (rice. 16.5), less, than the voltage at the poles of the source, t. e. in all of the external circuit. Note, that this is true only in the presence of current in the circuit. If the circuit is open, the potential of all points of a conductor, connected to one of the poles, will be the same.