ACTIVITY Law To measure resistance, voltag ic), current (ac) ar 0 measur , e (ac/dc), inuity of a ee (acc (ac) and check continuity of a given l APPARATUS AND MATERIAL REQUIRED A multimeter with test leads, three carbon resistors, a battery eliminator with tappings of 2 V,4V. and 6 V, a step-down transformer with out i : tput tappings of 6 V i bridge, a plug key, a microammeter, connecting ie Ahconae wae l THEORY MULTIMETER A multimeter is a widely used electrical instrument designed to measure directly the currents, potential differences or resistances over several ranges. Itis also known as AVO (ampere, volt and ohn) meter. Construction. A multimeter essentially consists of a sensitive moving coil galvanometer. The galvanometer's dial is calibrated in several ranges of amperes, volts and ohms. Fig. 4 Digital multimeter arallel Fig. 3 Analog multimeter sent values are connected in P: rent inA,mA or A. ‘are connected in series wit mV or V- and a number of jh the externally istances of diffe! table values of cur fferent values ter, shunt resi as to give suit resistances of di sitable values of To use multimeter as an ammet with the galvanometer coil, 80 Kime Oe potential differences in ; oe rd battery of 3 le, a battery i sistance measuring mo * wou set in toyed inside it. This battery sends current n th the To use mult galvanometer coil, so as When the multimeter is resistors get automatically coLAB MANUAL PHYSICS-XIl ected resistor whose resistance is being measured. The coil of the multimeter shows a deflag proportional to thisdirect current. The zeros of current and voltage scales are located on the extreme fet while the 20 of resistance scaleis located onthe extreme right This —— pe thei in the multimeter circuit is zero, current and hence deflection in the co oe tn The pin moves to the extreme right where the zero of resistance scale is located. As t he extemal resistance y bemensured increases the cvzentthrough the coll decreases and a smaller deflections produc, To make ac measurements with a multimeter, a junction diode rectifier is connected in the galvan. ‘meter circuit, The rectifier converts ac into de which produces a suitable deflection in the galvanomete, Some muitimeters have a single rotatory wafer switch which has enough wafers to selec difereny functions (chmmeter, milliammeter, de voltmeter or ac voltmeter) and the appropriate range fo each function. Some multimeters have two rotatory switches, one switch selects the meter function and the other selects the range. When the two probes ofthe multimeter are shorted, the multimeter should show 2er0 resistance. it does not show this, its pointer is brought to zero by using ‘zero Adj knob’ on the multimeter, Colour Code of Resistors. A colour cade is used to indicate the resistance value of a carbon resistor and its percentage accuracy. The colour code used throughout the world is shown in Table A3. Table A3 : Resistor colour code Letter as an aid to memory| _ Number re oe i a | er | 2 | o | fey Joa | } oe | 5 | Fos joe | vo} leeeco, |i: is wie | ow 2 How to remember colour code : BB ROY of Great Britain had Very Good Wife tiled og L bog + 01234 5 6 ee) 9 A setof coloured co-axial rings or bands is printed on the resistor which reveals the following facts: 1. The first band indicates the first significant figure Fist significant gure 2. The second band indicates the second significant figure, [i Second significant Figu* 3. The third band indicates the power of ten with which [r Decimal multiplet the above two significant figures must be multiplied [- ‘Tolerance to get the resistance value in ohms. 4. The fourth band inclicates the tolerance or possible —)) variation in percent of the indicated vy: alue. If the fourth band is absent, it implies a tolerance of 20% Fig. 5a. Section A : ACTIVITIES Illustration In Fig. 6, the colours of the four bands are yellow, violet, brown and gold ; the resistance value is Yellow Violet Brown Gold Y ‘ + + 4 7 1 £5% R=47x 10° 245%, [Lworxinc Diacrams Carbon resistor avec ate Black Red Red Black probe probe probe probe Multimeter ieads (oMultimeteras ohm meter (@)Maltimetr aa devolmeter_(@)Malimeter as ana olmeter Metre bridge fy 41S ww HA A Open +4 : Cs (¢ Multimeter for checking continuity ofa circu (@ Multimeter as an ac ammeter [Lbrocepure (a) Measurement of resistance 1. Mark the given carbon resistors as Ry, R, and Ry- ‘nd, rand Ath ring of each carbon resistor and record them in the ig. 7 Different uses of a multimeter. 2. Note the colours of 1st, observation table. 3. Using the colour code table for carbon resistors, determine the values of R,, R, and R, in ohms and write their tolerance limits. 4. To measure resistance with the multimeter, plug in the black and red probes in common (-) and positive (+) terminals respectively. Set the multimeter in resistance mode and select an appropriate range. Directly touch the ‘metallic ends of the two probes and adjust the zero adjustment knob so that the meter shows full scale deflection and reads zero ohm. tment has to be made each LESSER . ance scale of Casi outed time when we NOTE The zero adjust | different range. oe BassLAB MANUAL PHYSICS-XIl 0 the two ends of the carbon resistor Ryas the resistance scale. In case the reading. ince scale of smaller range. and Ry. Record the observations in 5, Separate the ends of the probes and connect them t shown in Fig. 7(a). Note the reading of the pointer on too small, do the activity again by choosing a resista 6. Repeat step 5 for the measurement of resistances R tabular form. (b) Measurement of de voltage ; as 7. Take a battery eliminator having variable de output in steps of 2 V- a 8. Set the selector switch of the multimeter to measure dc voltage. Select a range of 0-10 v, 9, Insert the black probe in common (-) socket and red probe in positive (+) socket of the multimeter 10. Switch on the battery eliminator and set its tapping at the minimum value, say 2V. 11. Touch and press the metallic end of the red probe with the positive terminal of the battery eliminator and that of black probe with its negative terminal. Note the reading. 12. Repeat steps 10 and 11 for tappings of 4 V and 6 V of the battery eliminator and note the reading for de voltage in each case. (c) Measurement of ac voltage 13. Take a step-down transformer having tappings of 6 V and 12 V. 14, Select the selector switch of the multimeter to measure ac voltage. Select a range of 0-10 V. 15, Insert the black probe in common (-) socket and red probe in positive (+) socket of the multimeter 16, Energise the transformer by connecting its input leads to ac mains. Select an output voltage of 6 Von it. Connect the leads of the multimeter to the output terminals of the transformer. Note the ac voltage on the multimeter scale. 17. Repeat the activity selecting a range of 0-50 V on the multimeter and setting the output of the step-down transformer at 12 V. Record the observations in a tabular form. (@) Measurement of ac current 18. Take a step-down transformer with its tapping at 6 V. 19, Select the switch of the multimeter to measure ac current and select a range of 0-250 mA. 20. Insert the black probe in common (-) socket and red probe in positive (+) socket of the multimeter. 21, Energise the transformer and connect the transformer, a resistance box and the multimeter in series. Introduce a resistance of 6 kQ from the resistance box and note the current in the multimeter. 22. Repeat the activity for resistances of § kQ and 10 kQ from the resistance box and record the values of current on the multimeter scale. (© To check continuity of metre bridge circuit 23. Insert the black probe in common (-) socket and red probe in red (+) socket 24. Set the multimeter in resistance mode and select a range of MQ. The cell within the multimeter automatically gets connected. 25. Set up an electrical circuit consisting of a metn ili i : a metre bridge, at ce Ra microammeter, and a plug key K, ame ee eee 26. Keeping the plug out of the key K, touch and press the metallic ends of the two probes at points A and B of the circuit full scale deflection indicates the continuity. 27, Similarly check the continuity between points B and C and then between points C and D. Full scale deflection (very low resistance) indicates continuity while zero deflection (high resistan®) indicates a discontinuity in the circuit, NOTE While checking the continuity, the battery ofthe circuit should be in switched off mode. 8Section A: ACTIVITIES [| onservarions (a) Measurement of resistance Range of resistance scale sel Least count of the scale ——2, "ected on the multimeter panel =0 toa. Table A4 : Measurement of resistance with multimeter Colour code of rings | Resin tance value and | peg Es Resistance measured 7 stance tolerance as i Diff es | td | dt | ath | perenour code | PY multimeter oO (Q) | Yellow | Viotet | Brown | Gata me & R, (b) Measurement of de voltage Range of voltage scale selected on the Least count of the scale =___V. multimeter panel =0 to Table AS : Measurement of de voltage with multimeter Output voltage marked Voltages measured by Difference S.No. on battery eliminator multimeter v-V, Yo (volt) V (volt) (Wolt} 1 2Vv 2 4v 3. 6V (©) Measurement of ac voltage Range of ac voltage selected on multimeter panel =0 to__V. Least count of the scale =___V. Table AG : Measurement of ac voltage with multimeter Output voltage marked on Voltages measured by Difference S.No. step-down transformer multimeter v-Vv, V,, (volt) ¥ (volt) (Wott) : 6V 2. 12V (@) Measurement of ac current Range of ac current scale selected on the multimeter panel =0 to_mA. mA. Least count of the scale = Table A7 : Measurement of ac current with multimeter Resistance inserted from the resistance box | Current measured by multimeter S.No. (KO) J (ma) 1 6kQ 2 8kQ 3. 10KLAB MANUAL PHYSICS-XIl (c) To check continuity between different points of a circuit Mo. Range of resistance scale selected on the multimeter penal =0 to Least count of the scale Q. Table A8 : Checking continuity of the circuit between different points SNo, | Checking continuity | Resistance reading | yature of resistance Nature of circuit | between pionts in multimeter 1 Aand B Very low/High | Contiriuous/Discontinuous 2 Band C Very low/High | Continuous/Discontinuous 3 Cand D Very low/High | Continuous/Discontinuous [Leesucr 1. Within the allowed limits of tolerance, the values of resistance measured by the multimeter are equal to the corresponding decoded values. 2. The values of dc/ac voltages measured by the multimeter are equal to the marked values indicated by the battery eliminator/step-down transformer. 3. The values of ac have been correctly measured by the multimeter. 4 As the multimeter shows zero resistance between different pairs of points on the given opin circuit, the circuit is continuous between those pairs of points. PRECAUTIONS 1. Instructions manual of the multimeter should be read thoroughly before handling it. 2. For measuring voltage, current and resistance, an appropriate selection of function switch and range switch should be made. 3. Red and black probes should be connected to the proper polarities for measuring de volge and current. 4. Whenever the meter is'used as ohm meter or switched to a new range, short-circuit its test Probes and set the pointer at zero with the zero adjustment knob. 5. Never use an ohm meter to measure resistance in an energised circuit. & Before using the meter for resistance measurement, check that the instrument cells a functioning properly. 7. If range of voltage, current or resistance to be measured is not known, start with maxim range and then go decreasing the range till easily observable deflection is obtained in 0 multimeter. [sources oF Error 2. The scale used for measuring voltage/current may not be proper. 2. Zero adjustment done for measuring resistance with analog multimeter may not be accu