#### Extremely small resistance calculations by Wheatstone bridge....

**Possible Experiments**
- The calculation of very small resistances using Wheatstone bridge
- The change of the resistance with wire radius and wire type

**Standard Equipment**
Resistance table, DC power supply, resistor set (0.4 ve 47ohm), multimeter, connection cables, wires: 0.2, 0.4, 0.6 mm Cr-Ni, 0.6 mm brass, 0.6 mm bronze.

**Technical Information **
The known resistance in the Wheatsone bridge is R, and the resistance to be measured is a thin wire. The wheatstoen table includes 5 wires connected. These are Cr-Ni wires of 0.2,0.4,0.6 mm radii and brass and bronze wires of 0.6 mm radius. In addition another Cr-Ni wire of 0.4 mm and 1 m of length is used as reaosta since it has a movable head. If the distances on the left and right of this head are L_{1}, L_{2} these correspond to the resistances R_{1} and R_{2} since resistance is linearly dependent on the wire length. These lengths can easily be measured from the ruler on the table. The resistance of a wire is given by the equation: R=rL/A where r is the resistivity, L is the length and A is cross section area of the wire. The Wheatstone connection is made such that four resistances are known R, R_{x} to be measured (one of the wires) and R_{1} and R_{2} are the left and right side reosta wires. After connection is done, the reosta head is moved until the multimeter connected to the middle of R-R_{x} and R_{1}-R_{2} reads zero current. At this adjustment, the lengths are measured and the resistance to be measured is calculated from R_{x}=RL_{1}/L_{2}.