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Explain how a moving - coil galvanometer is converted into a voltmeter . Derive the necessary formula. |
Answer» Solution :A moving - coil galvanometr is converted into a voltmeter by increasing its effective resistance by connecting a high resistance `R_(s)` in series with the galvanometer,as shown . The series resistance is also USEFUL for changing therange of any given voltmeter. Let G be the resistance of the galvanometer coil and `I_(s)` be the current required for full - scale deflection. Let V bethemaximum p. d. tobe measured. THEVALUE of theseries resistance `R_(s)` should besuch that when the p.d. applied across theinstrument is V, the current through the galvanometer is `I_(g)`. In HTE series combination,thep.d. V gets DIVIDED across thegalvanometer ( resistance, G) and the resistance ` R_(s)` : ` V = I_(g) G + I_(g)R_(s) =I_(g)(G+R_(s))` ` :. R_(s) = V/I_(s) - G` This gives the required value of theseries resistance. Thescale of the galvanometer is then calibrated so as to READ thepotential difference in vot or its submultiples directly. |
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