Theory - 7 :- Meter Range And Measure Types

 Here is a more detailed look at the theory and operation of ammeters, voltmeters, and ohmmeters.

Ammeter ⚡

An ammeter's fundamental principle is that it must measure the flow of charge (current) without impeding it. This is why it's connected in series and has extremely low internal resistance. 

• How it Works: Most analog ammeters are built from a galvanometer, which is a sensitive device that measures small electric currents. A galvanometer works by using a magnetic field. When current passes through a coil in the magnetic field, it creates a torque that causes a pointer to move. The deflection of the pointer is directly proportional to the current flowing through the coil.

• Range Extension (The Shunt): Since a galvanometer can only handle a very small current, a low-resistance wire called a shunt is connected in parallel with it. This shunt acts as a bypass, diverting the majority of the current around the delicate galvanometer. The ammeter's scale is then calibrated to show the total current flowing through both the shunt and the galvanometer, effectively extending its measurement range. The shunt's resistance is carefully chosen to ensure the ammeter reads accurately for a specific range.

Voltmeter 🔋

A voltmeter's primary role is to measure the electrical pressure difference between two points. To do this without altering the circuit, it must draw as little current as possible.

• How it Works: An analog voltmeter also uses a galvanometer. However, unlike an ammeter, a multiplier resistance (a very high-value resistor) is connected in series with the galvanometer. This series resistor limits the current flowing through the voltmeter to a very small, safe level.

• Range Extension (The Multiplier): The total voltage measured is the sum of the voltage drop across the multiplier and the small voltage drop across the galvanometer. The meter's scale is calibrated to read this total voltage. By using different multiplier resistors, you can create a multi-range voltmeter. For example, to measure a higher voltage, a larger multiplier resistor is used. This is why an ideal voltmeter has infinite resistance—it's designed not to disturb the circuit it's measuring.

Ohmmeter Ω 

An ohmmeter is unique because it's an active device; it contains its own power source (a battery) to perform a measurement. It does not measure resistance in a live circuit.

• How it Works: The ohmmeter's internal battery provides a constant voltage (V) to the component you're testing. The ohmmeter then measures the resulting current (I) that flows through the component. Using Ohm's Law ($R=V/I$), the meter's internal circuitry calculates the resistance (R) and displays the value.

• Scales and Types:

  • Analog Ohmmeters: These meters have a non-linear, "backwards" scale. When the test leads are shorted (0 Ω), the current is at its maximum, causing the pointer to deflect to the far right. When the leads are open (infinite resistance), no current flows, and the pointer stays at the far left. The scale is compressed at the high-resistance end and expanded at the low-resistance end.

  • Series Ohmmeter: This type is for measuring high resistances and connects the unknown resistance in series with the meter.

  • Shunt Ohmmeter: This type is for measuring low resistances and connects the unknown resistance in parallel with the meter.

• Key Consideration: The most crucial rule for an ohmmeter is to never use it on a powered circuit. The external voltage would interfere with the meter's internal battery, leading to inaccurate readings and potential damage to the instrument.