PhysicsLAB: Wheatstone Bridge

The purpose of this lab is to use a Wheatstone Bridge to investigate the resistance of resistors wired in series and in parallel.

Equipment

slide-wire Wheatstone bridge
3 100-W resistors
1 500-W resistor (to be used as R₁ )
2 50-W resistors
2 uncovered alligator clips
1 galvanometer
1 DC 6V power supply
leads and test probe

Procedure

Initially, set-up your circuit as shown in the diagram below. Be careful to not accidentally release the slide wire - it is very difficult to reconnect! Since your leads have alligator clips, merely clamp them over the screw heads.

Do NOT turn on the power supply until Mrs. Colwell has "okayed" your equipment. PLEASE, handle the resistors carefully - do not twist their leads, they break off VERY easily. The alligator clips are to help you "connect" the resistors into each of the combinations listed in the data chart on page 2.

Kirchoff's Loop Rule states that the sum of the voltage changes around a CLOSED loop in a circuit equals zero. Using this rule generates the following equations which when solved simultaneously can be used to determine an experimental value for the unknown resistance.

Combinations

Data Table:

Equilibrium
Position

Left Length
R₂

Right Length
R₃

Experimental
R_x

Theoretical
R_x

Percent
Error

A

B

C

D

E

F

Conclusions:

1a. Which combination (A through F) represented the largest resistance?

1b. Which combination (A through F) represented the smallest resistance?

2. Does combining resistors in series increase or decrease their effective resistance?

3. Does combining resistors in parallel increase or decrease their effective resistance?

4. In general, were your percent errors larger or smaller when the null point was located in the middle third (35 cm - 65 cm) of the slide wire?

5. Why do you think this result was true?

6. Why was it important to only "tap" the test probe to the slide wire and not "hold it down" for extended periods of time?

7. Why is this technique of measurement called a "null method?"