# Wheatstone Bridge

5. Jan 2020
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### Wheatstone Bridge

• 1. CERTIFICATE This is to certify that NISHCHAY SAXENA, a student of class XII-A has successfully completed the project under the guidance of Mr.Mukesh Shrimali (Subject Teacher) During the academic year 2019-20 in partial fulfillment of Physics practical examination conducted by CBSE. Signature of external examiner Signature of chemistry teacher Signature of principal
• 2. ACKNOWLEDGEMENT In the accomplishment of this project successfully, many people have best owned upon me their blessings and the heart pledged support, this time I am utilizing to thank all the people who have been concerned with project. Primarily I would thank god for being able to complete this project with success. Then I would like to thank my principal Mrs.Shalu Babbar and physics teacher Mr.Mukesh Shrimali, whose valuable guidance has been the ones that helped me patch this project and make it full proof success his suggestions and his instructions has served as the major contributor towards the completion of the project. Then I would like to thank my parents and friends who have helped me with their valuable suggestions and guidance has been helpful in various phases of the completion of the project.
• 3. AIM To study the construction and application of Wheatstone Bridge
• 4. CONTENTS 1. Introduction 2. Components of Wheatstone bridge 3. Circuit Construction 4. Working principle of Wheatstone bridge 5. Example Circuit 6. Applications of Wheatstone bridge 7. Limitations of Wheatstone bridge 8. Limitations of Wheatstone bridge 9. Summary 10. Bibliography 11. Precautions
• 5. INTRODUCTION Samuel Hunter Christie invented the Wheatstone bridge in the year 1833, which became popular with the works of Sir Charles Wheatstone in 1843. An electrical circuit that is set up to measure the unknown value of a resistor and creates a balance between the two legs of the bridge circuit is called a Wheatstone Bridge. As shown in the figure below, three resistances are known (one is variable/adjustable) and the fourth one has to be found out. Compared to the other measuring instruments such as voltage divider, the concept of Wheatstone bridge is widely used because of the accuracy in its measurement of resistance.
• 6. Components of Wheatstone Bridge  A resistor with an unknown resistance value.  Two resistors (with known resistance value)  Variable Resistor (a device like Rheostat or Preset could work)  Voltage/DC source  Galvanometer (or any device which indicates the voltage difference or the flow of current)  Connecting Wires  Circuit Construction  Construction of Wheatstone Bridge
• 7. CIRCUIRT CONSTRUCTION A Wheatstone bridge is a bridge-type structure having four resistors, three of known and one of unknown value. Here R1, R2, and R3 have known values among which R2 is adjustable and finally Rx is the value to be measured. Along with these resistances, a galvanometer (Vg) is there between B & D, and a DC supply between A & C.
• 8. WORKING PRINCIPLE OF WHEATSTONE BRIDGE Now according to the Wheatstone bridge principle if the ratio of the two resistances (R1/R2) on one edge is equal to the ratio of the two resistances (R3/Rx) on another edge then there will be no flow of current between the midpoints of the two edges of resistance. This condition of the bridge is known as the Balanced Bridge Condition. In the Balanced Bridge condition, the current through the galvanometer is zero and also the voltage difference between the points B & D becomes zero, i.e., at both points voltage level would be the same.
• 9. Writing equations for the balanced bridge condition would look like: R1/R2 = R3/Rx (or) R1 * Rx =R2 * R3 Thus, Rx = R3 * (R2/R1) This detection of zero current in galvanometer is of high precision, thus depending on the level of precision of known values, the unknown resistance can be found with the highest rate of accuracy and precision. In the Wheatstone bridge experiment, one resistor should always be variable in order to obtain a balanced condition. The Circuit performs at its best when the regulated voltage source is used, instead of the current with repelling characteristics.
• 10. EXAMPLE CIRCUIT Let us consider the below circuit where the bridge is in an unbalanced condition and we need to calculate the voltage difference between Q1 and Q2, i.e., Volt and hence the value of R4 needed to make the bridge balanced.
• 11. Example of Wheatstone Bridge As per the voltage division law, Vq1 = (R3/(R3+R1)) * Vs ,where Vs =100volts (voltage source) Putting values of R3 = 40 ohms, R1=50 ohms, and Vs= 100 volts, we get Vq1 = 44.4 volts Similarly, Vq2 = (R4/(R4+R2)) * Vs putting the values, R4 =50 ohms, R2 =100 ohms, and Vs =100 volts, we get Vq2 =33.3 volts Thus, Volt can be found as, Volt = Vq1 – Vq2 So, Volt = 44.4 – 33.3 = 11.1 volts
• 12. Now to make the bridge balanced, we can find a suitable value for R4 as done below: R4 = R2 * (R3/R1) putting the values of R1, R2, and R3, we have R4 = 100 * (40/50) = 80 volts Therefore, R4 = 80 volts is the value of resistor which should be used to make the bridge in a balanced condition.
• 13. APPLICATIONS OF WHEATSTONE BRIDGE  Used in Light detecting devices.  For measuring the changes in the pressure.  For measuring the changes in the strain of the circuit.  Used for the Sensing of mechanical and electrical quantities.  Also, photo resistive devices use this circuit.  Thermometers also use Wheatstone bridges for the temperature measurements which need to be accurate.  Values like capacitance, inductance, impedance, etc. can be measured with some variations in the Wheatstone bridge circuit.
• 14. LIMITATIONS OF WHEATSTONE BRIDGE Along with all these advantages, there are a few limitations of the Wheatstone bridge as well, such as:  Readings may be inaccurate under unbalanced conditions.  The range of measured resistance varies from a few ohms to mega ohms.  Susceptibility for high dc current is not there.
• 15. SUMMARY  Created in 1833, popularized in 1840s  Wheatstone bridges are one of the best methods of measuring resistance due to the basic mathematical ratio involved.  Accurate standards with sensitive enough voltmeter, measurements of resistance within .05% can be reached.  Many calibration laboratories still use this method today.  The Wheatstone Bridge are replaceable; however, for its simplicity and versatility the circuit is an indispensible piece of technology
• 16. BIBLIOGRAPHY WEBSITES- www.google.com www.wikipedia.com www.scribd.com Books- NCERT CLASS 12th Physics PRACTICAL Physics CLASS 12th
• 17. PRECAUTIONS  Clean the connecting wires properly.  Hold the jockey perpendicular to the wire.  All connections should be neat and tight.