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Explain how value of unknown resistor can be obtained by using meter bridge. |
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Answer» Solution :In one gap of meter bridge unknown resistor R and in second gas resistance S from resistance box is connected. Jockey key is SLIDED (moved) on wire AC. `rArr` ASSUME that when jockey key is on position D, galvanometer shows zero deflection. `rArr` Let length from A to D is l. `rArr` Resistance of AD wire = `R_(cm)` lwhere `R_(cm)`is resistance of wire per unit length (1 cm). `therefore ` Resistance of DC wire = `R_(cm) ` (100 - l) `rArr` Wire AB, BC, DA and CD form Wheatstone bridge, where, AB = R, BC = S, DA = `R_(cm)` l CD = `R_(cm) (100 - l) ` From principle of Wheatstone bridge for balancing of Wheatstone bridge, ` (AB)/(BC) = (DA)/(CD)` `therefore (R)/(S) = (R_(cm)l)/(R_(cm) (100 - l))` `therefore (R)/(S) = (l)/(100 -l)` `rArr` Unknown resistance can be obtained by using equation R= `S (l)/(100 - l)`. `rArr` For different value of S, value of unknown resistance R can be obtained by taking average value error can be eliminated. `rArr` To eliminate end correction. position of resistance R and resistance S from box can be interchanged and null point can be obtainedand error can be reduced. `rArr` For balancing bridge, null point should be at 50 cm (between 40 cm and 60 cm) and percentage error can be minimized. `rArr` From value of resistance obtained by meter bridge temperature of unknown resistance can be obtained. `rArr` By using meter bridge value of SMALL resistance can be known. |
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