EXPERIMENT NO. (1)Use of Ammeter, Voltmeter and Use of Ammeter, Voltmeter and Ohmmeter

Object: 1-To be Familiar with voltage, current, and resistance measurements .

Apparatus: 2-a-D.C. power supply .b-Resistors (different ratings) .c-Resistance Box .d-Muitimeter .e-Rheostat (variable resistance ) .f-Connecting wires and board .

3- Theory: Electrical measuring instruments are used in the laboratories either with analogue or digital readings . For the analogue reading instruments , the operation depends on many physical effects, of electrical voltage / current , and generally classified according to which of these effects is utilized in their operation (Magnetic effect , Elector / dynamic effect , Elector / magnetic effect) . Various types of analogue ammeters and voltmeters are used for measurements such as moving Iron , Moving Coil , Induction type for A.C only , and Electrostatic type both for A.C/D.C voltmeter . The moving coil galvanometer can be used to construct ammeter , voltmeter and ohmmeter .

Ammeter: Is an instrument designed to measure the magnitude of the current .It is connected in series with the circuit in which the current is to be measured , fig . (1-1a) the internal construction of the ammeter is shown in fig . (1-1b) the shunt resistance (R sh), is very small , nearly zero (why?) .

(a)Connecting of ammeter for current measurement

(b)Ammeter circuit.Fig . (1-1) Basic Ammeter.

Voltmeter:Is an instrument designed to measure the magnitude of voltage levels in a network . The correct method . of measuring voltage is to place the voltmeter in parallel with the element across which the voltage is to be measured as shown in fig . (1-2a).The internal construction of the voltmeter is shown in fig . (1-2b). The series resistance , (Rs), is of high value (why?).

(a)Connecting of Voltmeter For voltage measurement.

(b)Voltmeter circuit .Fig . (1-2) basic Voltmeter.

Ohmmeter: Is an instrument designed to measure resistance in low , medium , and high ranges . It is different from ammeter and voltmeter in that , it will show full-scale deflection for zero ohms and no deflection for infinite resistance .

The zero adjust resistance (regulating resistor ) has to be adjusted for full-scale deflection .

Fig . (1-3) Basic Ohmmeter.

Multimeter: Since ammeters , voltmeters and ohmmeter use the same movement they are combined into one instrument called '' Multimeter '' which has setting dials for voltage , current and resistance , A.C/D.C .

Procedure :1-measure 2 , 5 , 10 volt from a D.C. power supply using the voltmeter ?2-Read the value of the resistors : 51ohm , 180ohm , 390ohm , 1kohm?3-connet the ciraint in fig (1) and read the current in ciraint ?

Discussion:

1-Comment on your measurements .2-why the shunt resistor of the ammeter is small?3-why the series resistor of the voltmeter is high?