2.6 MeterBridge

Meter Bridge is a device used to determine the resistance of resistors by using wheatstone's Principle. It consist of a 1 m long uniform resistance wired clamped between two thick metal strips as shown in fig. Two Gaps G1 and G2  are provide as shown . In one of the gaps say G1, the resistor of unknown value is connected and on the other gap G2, resistance R is connected. Using sliding material (Jockey), the current in the galvanometer can be made zero.This occur when the  Wheatstone's condition is maintained. That is
(X/P)=(R/Q).

Internal Resisitance of a Cell

Till now we are dealing with ideal cells in which there is no loss of energy. But in all practical cells, there is a small loss of energy , due to the finite resistance offered by electrons and electrolyte of the cell is called internal resistance of a cell. Due to the internal resistance offered by electrons and electrolyte of the cell.This resistance offered by the electrolyte of the cell is called internal resistance of a cell.

Due to the internal resistance of cell, the actual potential difference that can be obtained from a cell is always less than e.m.f and the amount potential difference which is unavailable to the external circuit is called lost volt.

Thus in a practical circuit the potential difference obtained across a resistor connected to a cell is only V=E-Ir, where r id internal resistance and I is the current drawn from the cell . The term Ir is the volt.

             The current drawn from a cell depends on external resistance. Therefore the terminal voltage available depends on external resistance connected as well as internal resistance.Also internal resistance of  a cell varies with the amount of current drawn from the cell . In short terminal voltage as well as internal resistance depends on the external load connected to the cell.