Ohmmeter Calibration and Circuit Analysis Techniques

Why Must an Ohmmeter be Calibrated When Changing Ranges?

What is Calibration?

An ohmmeter must be calibrated each time you change the measuring range to verify the proper operation of the instrument. When calibration is not performed, the needle may deflect incorrectly on the ohmmeter scale. Calibration involves measuring a known resistance standard and comparing it with the ohmmeter reading. This should be done at several points on the instrument's scale and for various scales.

Connecting an Ammeter to Measure Current Through a Resistor

Series vs. Parallel Connection

In which of the two forms mentioned would you connect an ammeter to measure the intensity of current flowing through resistor R? Justify your answer.

The correct way to connect an ammeter is in series, not in parallel. An ammeter must be connected after the resistance so that all the current flowing out of the resistor passes directly through the ammeter. If you connect the ammeter in parallel, it would only measure part of the main current, not the current passing through the resistance.

Equivalent Resistance in Series Circuits

Deriving the Formula

1. Deduce the expression for the equivalent resistance in series:

R_S = R₁ + R₂ + ... + R_N = ΣR_i

Suppose a closed circuit with n resistors connected in series.

... I.R2 - Ei.R1 - i.Rn = 0, i (R1 + R2 + R3 ... + Rn) = E

i = E / (R1 + R2 + R3 ... + Rn)

For the equivalent resistance R_EQ, I = E / Req. Matching the last two, we have:

R1 + R2 + R3 ... + R = Req

Equivalent Resistance in Parallel Circuits

Deriving the Formula

Derive the expression for the equivalent resistance in parallel:

1/Req = 1/R1 + ...

Assuming a closed circuit:

i1 = V/R1. i2 = ...

V being the potential difference, the total current i is found by applying the theorem of the node to node (assuming two nodes at the ends of the FEM), and

I = i1 + i2 + ... in = V (1/R1 + ...)

If using the equivalent resistance R = V / Req combined the last two, we have:

Understanding Rheostats

Operation as Voltage Divider and Current Regulator

1. What is a rheostat? Explain its operation as a voltage divider and a current regulator.

A rheostat is an instrument used to alter the resistance values of conductors and thus induce changes in current. Its technological application offers significant advantages, allowing intensity variations in electrical circuits.

• As a voltage divider: The circuit device is incorporated only through terminals 1 or 2 and terminal 3.
• As a current regulator: Terminals 1 or 2 are connected to the power source, and terminal 3 and one of terminals 1 or 2 are connected to the circuit.

Measuring Voltage with an Ammeter and Ohmmeter

Indirect Measurement Technique

1. How would you measure the voltage across a resistor if you only have an ammeter and a tester which works only on the resistance scale?

First, measure the resistance with the tester. Then, connect the ammeter in series to measure the current. Finally, calculate the potential difference using Ohm's law: V = iR.