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www.GOALias.blogspot.comarrangement is schematically shown in Fig. 4.25. The scale of thisammeter is calibrated and then graduated to read off the current valuewith ease. We define the current sensitivity of the galvanometer as thedeflection per unit current. From Eq. (4.38) this current sensitivity is,φ NAB = (4.39)I kA convenient way for the manufacturer to increase the sensitivity isto increase the number of turns N. We choose galvanometers havingsensitivities of value, required by our experiment.The galvanometer can also be used as a voltmeter to measure thevoltage across a given section of the circuit. For this it must be connectedin parallel with that section of the circuit. Further, it must draw a verysmall current, otherwise the voltage measurement will disturb the originalset up by an amount which is very large. Usually we like to keep thedisturbance due to the measuring device below one per cent. To ensurethis, a large resistance R is connected in series with the galvanometer.This arrangement is schematically depicted in Fig.4.26. Note that theresistance of the voltmeter is now,R G+ R ; R : largeThe scale of the voltmeter is calibrated to read off the voltage valuewith ease. We define the voltage sensitivity as the deflection per unitvoltage. From Eq. (4.38),φ ⎛NAB ⎞ I ⎛NAB⎞ 1= ⎜ =V ⎝⎟k ⎠⎜ ⎟V ⎝ k ⎠ RMoving Charges andMagnetism(4.40)An interesting point to note is that increasing the current sensitivitymay not necessarily increase the voltage sensitivity. Let us take Eq. (4.39)which provides a measure of current sensitivity. If N → 2N, i.e., we doublethe number of turns, thenφ φ→ 2I IThus, the current sensitivity doubles. However, the resistance of thegalvanometer is also likely to double, since it is proportional to the lengthof the wire. In Eq. (4.40), N →2N, and R →2R, thus the voltage sensitivity,φ φ→V Vremains unchanged. So in general, the modification needed for conversionof a galvanometer to an ammeter will be different from what is neededfor converting it into a voltmeter.FIGURE 4.25Conversion of agalvanometer (G) toan ammeter by theintroduction of ashunt resistance r sofvery small value inparallel.FIGURE 4.26Conversion of agalvanometer (G) to avoltmeter by theintroduction of aresistance R of largevalue in series.Example 4.13 In the circuit (Fig. 4.27) the current is to bemeasured. What is the value of the current if the ammeter shown(a) is a galvanometer with a resistance R G= 60.00 Ω; (b) is agalvanometer described in (a) but converted to an ammeter by ashunt resistance r s= 0.02 Ω; (c) is an ideal ammeter with zeroresistance?EXAMPLE 4.13165