AIM

To calibrate voltmeter and ammeter using student’s potentiometer

ARPIT BHARDWAJ(ROLL NO – 232/co/11) ASEEM GOYAL(ROLL NO-233/co/11)ASHAT MEHTA(ROLL NO – 234/co/11)

LIST OF EXPERIMENTS

1. To determine the current sensitivity of a ballistic galvanometer and it’s resistance by half deflection method.

2. To calibrate the given voltmeter and ammeter using student’s potentiometer

3 To study the sparkling potential of a given neon lamp.4. To determine the resistivity of a semiconductor using

four probe method5. To determine the high resistance by leakage method.6. To determine the band gap of a given specimen.7. To determine the susceptibility of MnCl28. To study the Hall Effect9. To determine Planck’s constant.

Apparatus Required :

Galvanometer Voltmeter Ammeter Standard Cell Student’s Potentiometer Rheostats (150 Ω,10 Ω, 10 Ω) 1 ohm Resistance Power Supply Connecting Wires

INTRODUCTION

CALIBRATION

Calibration is the validation of specific measurement techniques and equipment. At the simplest level, calibration is a comparison between measurements one of known magnitude or correctness made or set with one device and another measurement made is as similar a way as possible with a second device

Galvanometers

Galvanometers are instruments which are intended primarily to indicate the existence of a current and which may under circumstances, be capable of measuring it. The construction of the galvanometers is based on the interaction between coils carrying currents and magnets.

Ammeters

Ampere meters ,or ammeters are instruments used for measurement of current in amperes. To measure a current, an ammeter is inserted in the circuit so that the whole current to be measured passes through it. The instrument must be able to record the value of current without changing it. Ammeter should be a low resistance instrument. Ammeter should have zero resistance so that there is no potential drop across it and the current is not altered

Voltmeters

A voltmeter measures the change in voltage between two points in an electric circuit and therefore must be connected in parallel with the portion of the circuit on which the measurement is made. It is necessary for the voltmeter to have a high resistance so that it does not have an appreciable affect on the current or voltage associated with the measured circuit

Potentiometer

A potentiometer is a variable resistor or voltage divider

Usually there is a three terminal resistor where the center connection is manipulated. If two terminals are used it acts as variable resistor. If all three are used then it acts as a variable voltage divider.

The potential difference is a portion of potentiometer wire between the ends of jockey at the time of balancing is V=KL, where K is the potential gradient of the wire

The potentiometer is the most accurate device for measurement of emf. Its accuracy depends upon the constancy of cross section of wire and the uniformity of cross section of wire

THEORY

g

R(1 ) R(2)

B(1)

1.018

B(2)

A

E1

E2

1ohm

Calibration of Ammeter

A

K

X + X’-

RhB(2)

Ammeter and Rheostat

Calibration of Voltmeter

VX+ X’-

Q

B(2)

K(P)

Let a constant P.D. be maintained between the two end A & B of potentiometer wire with a at being higher potential than B. Let + pole of cadmium cell D be connected to end A & through galvanometer to jockey of potentiometer. Then if l1 be the length of potentiometer wire for no deflection in galvanometer , we have

E=Kl1 , where E is the emf of cadmium cell & K is potential gradient of potentiometer .This gives value of K

Calibration of Ammeter

Now let cadmium cell be replaced by a one ohm standard resistance through which a steady current is flowing.

Then if l2 be the length of potentiometer wire for no deflection in the galvanometer the P.D. between the terminals of the standard one ohm resistance is given by:

V=Kl2

Let reading of ammeter be I(o). The graph between error(I(o)-I) versus I(o) is known as calibration curve of ammeter

Calibration of Voltmeter

Now let cadmium cell be replaced by portion MP of a rheostat MN through whole of which a steady current is maintained.

Then if l2 be the length of potentiometer wire at the null point, the P.D. across the portion MP of the rheostat is given by:

V=kl2

Let the reading of voltmeter be V(o). Then the graph V(o)-V versus V is known as calibration curve of voltmeter

Voltmeter and Galvanometer

Procedure

Step 1

Use the experimental arrangement for the calibration of potentiometer wire, using the standard cell e s.

Start with your sliding contact C near the centre of the bridge.

Press the contact C. The galvanometer will probably deflect. Find a point C for no deflection

Now close switch K1 and again adjust C for no deflection.

Record the final setting of the contact point C and known value of the emf of the standard cell

Step 2

Turn the voltage switch to E2.

Plug the key K(A) and pass a suitable current in the circuit by adjusting rheostat.

Locate the balance length L by adjusting the coil knob to a suitable number and then sliding the jockey on the stretched wire. The ammeter gives the observed value of current.

Calculated value of the current I(c) is found as follows:

I(c)=I(A)x10-3

Step 3

Plug the keys and by adjusting the coil knob and by adjusting the sliding contact of the potential divider device.

Bring a suitable voltage reading in the voltmeter V.

Voltmeter shows the potential difference across the rheostat resistance. This is V(o).

V(c)=l(v)x10-3

Observation

For Ammeter

S.No. BalancingLength(cm)l

Calculated value of current(A)Ic

Observed value of Current(A)I0

Error(Ic- Io)

1 85 0.085 0.10 0.015

2 137 0.137 0.15 0.013

3 173 0.173 0.20 0.037

4 225 0.225 0.25 0.025

5 260 0.260 0.30 0.040

For Voltmeter

S.No. Balancing length(cm)l

Calculated value of voltage(v)Vc

Observed value of voltage(v)Vo

Error(Vc-Vo)

1 132.2 0.132 0.1 0.032

2 245.9 0.245 0.2 0.045

3 348.4 0.348 0.3 0.048

4 449.0 0.449 0.4 0.049

5 569.5 0.569 0.5 0.069

For Ammeter

For Voltmeters

Result

Hence the given voltmeter and ammeter have been calibrated & the calibration curve of ammeter and voltmeter is obtained.

Precautions

Keep the voltage of E1 & E2 equal for better results

Do not touch the rheostats once set. Do not run the current for long

durations. Take out the key once reading is

taken.

Advanced Applications

The student’s potentiometer is designed to measure emf up to 1.6 volts. The unknown voltage is connected across a series of resistors and a known fraction of total unknown voltage is measured by the potentiometer. By multiplying the potentiometer reading by the appropriate volt box factor, the unknown voltage can be determined.

Voltage Measurement

Current Measurement

The potentiometer is designed primarily to measure voltage. But by applying Ohm’s law, the current may also be measured with a potentiometer. That is, knowing the precise value of a resistance R and the precise voltage measured across it, the current which produces this voltage drop can be determined.

Wheatstone Bridge

Special binding posts permit making independent connections. Therefore, the calibrated slidewire of the student’s potentiometer can be used as two arms of a wheatstone bridge.

The slidewire is calibrated in 100 uniforms divisions over its full travel, therefore the reading in the window can be used to determine the ratio between these arms of the bridge

Hydrogen Ion Measurement The potentiometer is commonly used to

measure the emf generated between hydrogen and calomel cells in the determination of hydrogen ion concentration. Since the solution is usually high in resistance, the voltage measurement would be in error if current is drawn. Therefore, the main advantage of potentiometer is that the potentiometer balance draws no current from the cell