# Experiment using a Wheatstone Bridge

• Taniaz
In summary, the student is trying to find the unknown resistance using a Wheatstone bridge. They connected the unknown resistance wire in the right gap and they are getting a resistance of approximately 2.2 ohms. However, they are unsure if this is accurate because the wire's length and resistivity are too small. They should verify this resistance with a digital multimeter.
Taniaz

## Homework Statement

Determine the unknown resistance using a Wheatstone Bridge.

## Homework Equations

Unknown Resistance = known resistance (100-Length) / Length

## The Attempt at a Solution

I've connected the set up as shown in the pictures but my only concern is, I don't have a resistance box so I'm using those blue boxes which seem like a resistance box to me but with just one resistance instead of many. I connected the wire of unknown resistance as shown, I stripped off the insulation of the copper wire and then wrapped it around. The null point I'm getting it not in the middle, it's closer to the higher potential and I get the resistance of the wire to be approximately 2.2 ohms when I use the 10 ohm blue resistance box. Is that correct?

Secondly, the circuit diagram provided to us also has a rheostat, what use is a rheostat in this experiment?

Thanks

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It is not very clear what you are doing .

Please post the circuit diagram that you have been given and show what components you have used in each part of the circuit .

Last edited:
The second picture shows the complete set up which most definitely resembles a wheatstone bridge. Two ends are connected to the power supply. Left gap to a known resistance, right gap to an unknown resistance and the connection in between the left and right gap is made to the galvanometer which is connected to the jockey. Switch on the power supply, touch the meter string on one end with a jockey, it should show a positive deflection of the galvanometer (the end at the higher potential) and the other end should show negative deflection. This will ensure your wheatstone bridge is working. Then find the null point where the deflection is zero and measure that length. Finally substitute it in the equation provided above to find the unknown resistance.

Is it this experiment?

cnh1995 said:
Is it this experiment?
View attachment 110258

Yes, but the problem as I was mentioning earlier is that we don't have a resistance box in the lab. So we used those little blue boxes as shown in the picture which are we're guessing fixed resistance boxes in place of the known resistance and a copper wire for the unknown resistance.

Taniaz said:
Yes, but the problem as I was mentioning earlier is that we don't have a resistance box in the lab. So we used those little blue boxes as shown in the picture which are we're guessing fixed resistance boxes in place of the known resistance and a copper wire for the unknown resistance.
Why are you using two blue boxes in the setup in the second image? You only need one, in the left gap. Have you connected the unknown resistance wire in the right gap? Is that the right gap in the last image?

cnh1995 said:
Why are you using two blue boxes in the setup in the second image? You only need one, in the left gap. Have you connected the unknown resistance wire in the right gap? Is that the right gap in the last image?

Yes I was only using it to test whether I get the other resistance or not. I later replaced it with a copper wire, yes I connected it in the right gap. So the blue box is fine?

Secondly, they suggest we should use a rheostat in the set up, what use will a rheostat be? Thanks

Taniaz said:
So the blue box is fine?
Yes. The null point should be on the right side of the meter scale, at about 18cm from the right end if you are getting 2.2 ohms as the unknown resistance.
Taniaz said:
The null point I'm getting it not in the middle, it's closer to the higher potential and I get the resistance of the wire to be approximately 2.2 ohms when I use the 10 ohm blue resistance box. Is that correct?
Looks ok but verify it with a digital multimeter. I think the length and resistivity of the wire are too small to have a resistance of 2.2 ohm. I may be wrong, I'm not sure.

Taniaz said:
Secondly, they suggest we should use a rheostat in the set up, what use will a rheostat be? Thanks
I see nothing special in it.
Using a rheostat, you can take several readings for the null deflection point by varying the rheostat's position and find the unknown resistance using each reading. The final value for the unknown resistance will be the average of all the calculated values. This method will be more accurate.

cnh1995 said:
Yes. The null point should be on the right side of the meter scale, at about 18cm from the right end if you are getting 2.2 ohms as the unknown resistance.

Looks ok but verify it with a digital multimeter. I think the length and resistivity of the wire are too small to have a resistance of 2.2 ohm. I may be wrong, I'm not sure.

Should I use higher known resistances or 10 ohms is fine? I have another box of 5 ohms and 20 ohms so I can use those too or I have a colour band resistor I can use by attaching it with crocodile clips. Thanks

Taniaz said:
Should I use higher known resistances or 10 ohms is fine? I have another box of 5 ohms and 20 ohms so I can use those too or I have a colour band resistor I can use by attaching it with crocodile clips. Thanks
Well, I think you should use even smaller known resistance (5 ohm would work) for more accuracy in this case since the unknown resistance is very small.

## 1. What is a Wheatstone Bridge?

A Wheatstone Bridge is a type of electrical circuit used to measure unknown resistance values by comparing them to known resistances. It consists of four resistors connected in a diamond shape with a voltage source and a galvanometer.

## 2. How does a Wheatstone Bridge work?

A Wheatstone Bridge works by using the principle of null deflection, where the galvanometer reads zero when the bridge is balanced. By adjusting the known resistances, the unknown resistance can be calculated using the bridge equation.

## 3. What are the advantages of using a Wheatstone Bridge?

One advantage of using a Wheatstone Bridge is its high accuracy in measuring resistance. It is also able to measure a wide range of resistances, making it versatile for different experiments. Additionally, it is a simple and inexpensive circuit to construct.

## 4. What are some common applications of a Wheatstone Bridge?

A Wheatstone Bridge is commonly used in strain gauge measurements, where it can detect small changes in resistance due to mechanical strain. It is also used in temperature and humidity sensors, as well as in circuit testing and calibration.

## 5. Are there any limitations to using a Wheatstone Bridge?

One limitation of using a Wheatstone Bridge is that it requires a stable power supply and can be affected by temperature changes. It also requires careful calibration and adjustments to ensure accurate measurements. Additionally, it is not suitable for measuring extremely low or high resistance values.

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