Wheatstone's Bridge

Whetstone’s bridge isthe most accurate method available for measuring resistances and is popular for laboratory use. The circuit diagram of a Wheatstone bridge is given in Fig. 11.1. The source of emf and
switch is connected to points A and S, while a sensitive current indicating meter, the galvanometer, is connected to points C and D. The galvanometer is a sensitive micro ammeter a zero center scale. When there is no current through the meter, the galvanometer pointer rests at 0, i.e. mid scale. Current in one
direction causes the points deflect on one side and current in the opposite direction tothe other side.When SW1 is closed, current flows and divides into the two arms at point A i.e. I1and I2. The bridge is balanced when there is no current through the galvanometer, or when the potential difference at points C and D is equal, i.e. the potential across the galvanometer is zero. To obtain the bridge balance equation, we have from the fig 4.1 For the galvanometercurrent to be zero, the following conditionsshould be satisfied

This is the equation for the bridge to be balanced. In a practical Whetstone’s bridge, at least one of the resistance is made adjustable, to permit balancing. When the bridge is balanced, the unknown resistance (normally connected at R4) may be determined from the setting of the adjustable resistor, which is called a standardresistor because it is a precision device having very small tolerance

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Applications of Wheatstone bridge

1. The basic application of a Wheatstone bridge ismeasurement of resistance.It is used to measure
medium resistance values.
2. It can also be used to measure inductance and capacitance values.
3. Various industrial applications involve measurement of physical quantities (such as temperature, pres-sure, displacement etc) in terms of electrical resistance. The various industrialapplications in which a
Wheatstone bridge is used are.
(i) Temperature measurement systems involving electrical resistance thermometers as temperature
sensors.
(ii) Pressure measurement systems involving strain gauge as secondary transducer.
(iii) Measurement of static and dynamic strains.
(iv) It is usedwith explosive meter to measure the amount of combustible gases in a sample.
(v) Temperature measurement systems involving electrical resistance thermometers as temperature
sensors.
(vi) Pressure measurement systems involving strain gauge as secondary transducer.
4. Measurement of staticand dynamic strains.
5. It is used with explosive meter to measure the amount of combustible gases in a sample

Limitations of Wheatstone bridge

1. Wheatstone bridge is not suitable for measuring low resistances because the resistance of leads and
contacts of the bridge cause errorsin the value measured by the Wheatstone bridgeand thus affects the
measurement of low resistances.
2. Wheatstone bridge cannot be used for measurement of high resistance also, because a galvanometer is
not sensitive to the imbalance of the bridge caused bythe high resistance of the bridge. This problem
can be overcome byreplacing the galvanometer with a Vacuum Type Volt Meter (VTVM) and by
replacing the battery with a power supply.
3. A Wheatstone bridge cannot be used in high temperature or temperature-varying environment because
the resistance of the arms of the bridge changes due tochange in temperature.
4. The resistance of the bridge arms also changes due to heating effect of the current passing through
the resistance. Flow of very large current through the resistors leads to a permanent change of resistance
value .