# Measure resistance by comparison

According to Ohm's law, current through a conductor and the potential difference between it's end are proportional. The constant of proportionality is called the resistance. Mathematically R=(V)/(I). When two resistors are connected in series, the current will be same through both. I=(V1)/(R1)=(V2)/(R2). if the value of one resistance and the voltage across both are known, the other resistance can be calculated from R1=R2×(V1)/(V2)

# Objective

Find the value of an unknown resistance by comparing it with a known resistance, using the equations given above. Assume R1 is the unknown resistance and R2 is 1000Ω

# Procedure

- Fix the two resisters in series on a bread board.
- Connect the junction to A1
- Connect the other end of R2(1 kΩ) to Ground.
- Connect one end of R1 to PV1
- Set PV1 to 4 volts.
- Enable the Checkbutton on top right, to measure the DC voltage at A1.

Current I=(VA1)/(R2) and R1 can be calculated using R1=(VPV1−VA1)/(I).

# Ohm's law in AC circuits

It can be easily shown that this measurement can be done using AC also. We will use both A1 and A2 inputs here.

- Fix the two resisters in series on a bread board.
- Connect the junction to A2
- Connect the other end of R2(1 kΩ) to Ground.
- Connect one end of R1 to both WG and A1
- Set WG to 1000Hz
- Enable A1 and A2
- Enable the Cursor Check button to diplay the voltages at the cursor

Taking voltage reading from the picture below, I=(1.92)/(1000) and R1 =(3.01 − 1.92)/(0.00192)= 576.7

# Discussion

In this measurements we have made the assumption that no current flows in to A1 and A2. This is not true, they both have an input impedance of 1MΩ . This will matter when we use resistance values of mega Ohms range. To illustrate this connect WG to A1 using a wire and the same signal to A2 through a 1MΩ resistor. Try to explain the results using Ohm's law.