AVO-meter stands for Ampere, Volt and Ohm meter. AVO-meter is an electrical instrument which can measure the current in amperes, potential difference in volts and resistance in ohms.

 Avometer

Construction

Avometer basically consists of a sensitive moving coil galvanometer which can be converted into a

(i) multirange ammeter by connecting a current measuring circuit with it.

(ii) multirange voltmeter by connecting a voltage measuring circuit with it.

(iii) multirange ohmmeter by connecting a resistance measuring circuit with it.

Function Switch

For this purpose, a switch called function switch is used. FS is the function selector switch, which connects the galvanometer with the relevant measuring circuit.

Different parts of AVO meter

Following parts describe the working of AVO meter.

Voltage Measuring Part of AVO Meter:

The voltage measuring part of AVO meter is a multi range voltmeter. It consists of a number of resistances each of which is connected in series with the moving coil galvanometer with the help of switch which is called range switch.

The value of each resistance depends upon the range of voltmeter, which it controls; these series resistance are called multipliers. We can measure AC voltage with AVO-meter. For this purpose, AC is first converted into DC and then measured.

Can we measure AC voltage with AVO meter?

Yes, for this AC is first converted into DC and then measured.

Current Measuring Part of AVO Meter:

The current measuring part of -AVO meter is multi-range ammeter. It consists of a number of low resistances connected in parallel with the galvanometer. The value of these resistances depends upon the range of ammeter. This circuit also has range selection switch SR which is used to select the particular range.

Resistance Measuring Part of AVO Meter:

The resistance measuring part of AVO meter is multi range ohmmeter. It consists of a battery of emf Vo, and a variable resistance rs, connected in series with the galvanometer of resistance Re. When the function switch is switched to be position X3, then the circuit is connected with X, Y terminals, of AVO-Meter.

Before measuring the unknown resistance by ohm-meter it is first zeroed which means that we short circuit the terminals X, Y and adjust rs to produce full scale deflection.

Digital Multimeter (DMM)

It is an electronic instrument which is used to measure current, resistance and voltage in a circuit.

The important features of digital multimeter are:

(i) It is digital version of an AVO meter.

(ii) It is very accurate device.

(iii) It is very easy to operate.

(iv) It is much easier to read.

(v) It removes the reading error.

(vi) It shows the digital values with decimal point, polarity and the unit of V, A or Ω.