The resistance to current in a DC circuit is known as the load. Usually a load consists of not one but several different resistances, which are equivalent in their effect of a single resistor. But the size of this equivalent resistor depends on how the composition resistances are connected together.
Resistors in series and parallel
If a set of resistors are joined end to end they are said to be connected in series. The resistance of the equivalent resistor is found by simply adding together the component resistances. Whatever current flows out of one must flow into the next the current cannot alter or disappear unless the wire branches; the current in each resistor is thus the same.
If the connecting wires in a circuit branches, however, and the current flows along two separate paths through two different resistors before recombining, the resistors are said to be connected in parallel. When the current arrives at the branching point it divides and some flows down each branch. This is “easier” than forcing all the current through only one of the resistors, and as a result the equivalent resistance of the circuit is less than other of the component resistances. (The reciprocal of the equivalent resistance is equal to the sum of the reciprocals of the component resistance.) More complicated resistive circuits are a combination of resistances in series and resistances in parallel. And in all circuits the resistance of the connecting wires themselves contributes to the overall resistance of the circuit.
A potentiometer is a variable resistor, as used for volume control on a radio receiver. One form can be used in the laboratory for finding –or, more strictly speaking, comparing – the EMF of cells or other DC voltage sources. It consists of a battery driving a constant current around a circuit that includes a straight length of bare wire of high resistance. The potential (or voltage) across the wire decreases uniformly from one end to the other, therefore the
The Wheatstone bridge
Bridges are circuits used for finding (or comparing) resistances. The one improved upon by Charles Wheatstone in 1843 is particularly useful in physics experiments. A galvanometer forms a “bridge” between two parallel circuits, each containing two resistors. If the four resistances are chosen at random, a current flows through the galvanometer. But if their values are relate in a particular way, a current flows and a null reading is obtained on the galvanometer. The required relationship is that the ration of the bridge is the same.
Suppose only one of the resistances in one arm of the bridge is known, the ratio of the two resistances in the other arm of the bridge is altered until a null reading is obtained. This ratio is then the same as the ratio of the known resistance to the remaining, fourth resistance, whose value can thus be calculated. In summary, in order to find one of the resistances we need to know one of the other three plus the ratio (but not the actual resistances) of the remaining two. A form of the whetstone bridge circuit that satisfies these criteria is the meter bridge, which has a meter of high-resistance wire and movable contact (like a potentiometer) that forms two of the resistance.
Resistivity and conductivity
The resistance of a piece of material depends on its composition and its size. Consider a cylindrical conductor, with a current entering one end and leaving at the other. the wider the cylinder, the “easier” it is for the current to flow along it, and the lower is its resistance, (in the same way, more water flows through a wide pipe than through a narrow one.) also, the linger the cylinder, the grater is the potential difference needed to make a given current flow , and the higher the resistance.
Electrical wires are essentially of cylindrical shape. A thick, short wire has a lower resistance (and carries more current for a given potential difference) than a ling this wire. But a copper wire of a given size and shape has a lower resistance than a similar tungsten wire. Copper is the better conductor; it is said to have lower receptivity than tungsten in fact, only one –third as much. The reciprocal of resistivity is called the conductivity of the material. Copper has conductivity about three times that of tungsten.