The Circuit.—In connection with the manipulation of batteries, there is one important item to consider, viz. the resistance in the circuit, which may be divided into external and internal.
Resistances.—The external resistance in practice is that which exists in the conducting line, and the various instruments connected with it.
The internal resistance is that which exists in the battery itself. All known conductors oppose a sensible resistance to the passage of an electric current, and the strength of the current, or in other words, the quantity of electricity passing per second from one point to another, when a constant difference of potentials is maintained between them, depends on the resistance of the wire on the conductor joining them. A bad conductor does not let the electricity pass so rapidly as a good conductor, that is, it offers more resistance.
Resistance in a wire of constant section and material is directly proportional to the length, and inversely proportional to the area of the cross section.
The electrical resistance of a conductor must not be considered as analogous to mechanical resistance, such as the friction which water experiences in passing through a pipe, for this frictional resistance is not constant when different quantities of water are being forced through the pipe, whereas electrical resistance is constant whatever quantity of electricity be forced through the conductor.
Application of Ohm's Law.—Ohm's law, which governs the strength of the current, is expressed by the equation
| C = | E | or R = | E | or E = CR. |
| R | C |
- Where C is the strength of the current;
- E is the E.M.F. or difference of potentials;
- and R is the resistance of the circuit.
In words, Ohm's law means that the strength of the current is directly proportional to the E.M.F., and inversely proportional to the resistance of the circuit.
As before stated, the resistance of the circuit consists of an external and an internal resistance, therefore when these resistances are separately considered, the equation C = E / R must be converted into C = E / (x + r), where x is the external, and r the internal, resistance.