Selection criteria for an instrument | Multimeter | Analog multimeter | Voltage coil | Wattmeter | Ammeter | Voltmeter

Selection Criteria for an Instrument: 

The points to be considered for the selection of a measuring instrument are as follows:
1. Whether to measure an ac or dc quantity.
Quantity (current, voltage, power etc) to be measured.
2. Range of the instrument.
3. Types of instrument.
4. Accuracy
5. Resolution.

Multimeters :

The name of the instrument itself suggests that a multimeter is used to measure more than one quantities.
Usually multimeters measure the following quantities: a.c. or d.c. current. 2. a.c. or d.c. voltage Resistance.
In addition to this, it can also be used to test capacitors, i diodes and transistors.
Types of multimeters : The multimeters can be of two types:

1. Analog multimeters
2. Digital multimeters (DMM) 

Analog Multimeters :

Fig. 4.11.I shows an analog multimeter.
Multimeter is basically a moving coil instrument. A rectifier unit is also provided with the instrument.
It is a multirange instrument and various ranges are obtained by different resistance elements in seres or in parallel with the movement of the instrument. With the help of a rotary selector switch the vanous ranges are used.
Usually different ranges used are somewhat similar to thosei given below.
Current ranges : 0-IA. 0-5A, 0-10A Voltage rangesi : 0-10V. 0-50V. 0-150V, 0-500V, 0-1000V.
Resistance ranges : 0-19, 0-10Q, 0-100Q, 0-100002. Some multimeters are provided with an additional equipment known as a current transformer. With the help of this transformer currents upto 400 A are usually measured, Such a multmeter is known as a tong tester.
Mulumeter is a compact and portable instrument and therefore very popularly used in practice.

Voltage coll or pressure coll:

The coil shown between the terminals C (common) and V (voltage) in Fig. 4.8.2 is called as the voltage or pressure coil. It is always connected across the supply to measure thei voltage.
The resistance of voltage coil is large. It is made of thin wire with a large number of turns.

Wattmeter reading :

Let the current flowing through the current coil be I, and let the voltage across the pressure coil beV
Then the wattmeter reading is given by. neeosVne
.(4.8.1)
Ang.e between Vo.and c一Current through the current coil一Wltage across the pressure coil Wattmeter reading
(A-1677)
The wattmeter reading is proportional to the cosine of the angle between V- and L i.e. cos (VAL)
Wattmeter Connections :
Draw connection diagram of ammeter, voltmeter and watimeter with AC suppy
Fig. 4.8.3 shows the method of connecting a wattmeter. The current coil is connected in series with the load. i
The pressure coil is connected across the supply

 Fig. Wattmeter connections

Connection of Ammeter, Voltmeter and Wattmeter:

Fig. 4.8.4 shows the connection diagram showing the connections of ammeter, voltmeter and wattmeter in a single phase resistive circuit.

Application of wattmeter

1. Measurement of power
2. Measurement of voltage and current.