The Wheatstone Bridge is one of the methods of measuring the value of unknown resistance. The media given below describes the principle, working, and a practical application of the Wheatstone Bridge.
This bridge circuit is most sensitive when all four resistors have similar resistance values. However, the circuit works quite well in any event in practical resistance measurement instruments.
If Q can be varied over a 10:1 resistance range and P is of a similar value, we can switch decade values of S into and out of the circuit according to the range of value we expect from RX. Using this method, we can accurately measure any value of RX by moving one multiple-position switch and adjusting one precision potentiometer.
How to Find That No Current Flows Through a Bridge Connection?
An instrument, that can indicate the flow of even a few microamperes (millionth of an ampere), called galvanometer, is used. There are galvanometers that give full-scale deflection for 25 microamperes.
In the professional Wheatstone bridges, Rheostat R1 in series with a normally-closed Push Button P.B-2 is connected across the Galvanometer. This arrangement protects the Galvanometer when the bridge is in severely unbalanced condition causing very high current flowing through it and damaging the same.
In the case of excessive deflection, initial course adjustment of the variable resistor is done. Final and precise adjustment of the variable resistance is made keeping the shunt resistor of the galvanometer open. There are different types of measuring instruments to measure the value of the resistance values. 'Slide Wire Bridge' is one of such instruments.
Slide Wire Bridge
Slide Wire Bridge is a bridge circuit in which resistance in one or more branches is controlled by the position of the sliding contact on a length of resistance wire along a linear scale. It is used to balance the arms of the bridge. The bridge is said to be in a balanced condition when the value of slide wire resistance is such that no current flows in the galvanometer.
Resistance Pyrometers
The Resistance Pyrometers are used to measure high temperature. It can measure the temperature most accurately up to 150000C. The principle of operation of the resistance pyrometers is based on the fact that when a current is passed through a platinum coil, it gets heated.
Consequently, its resistance increases progressively with its temperature. The temperature of a hot location can be measured by determining the electrical resistance of a platinum coil placed in the hot place and comparing this with the resistance which is calibrated already at different temperatures. Resistance pyrometers do not respond easily to sudden changes in temperature.
