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Questions
State how a moving coil galvanometer can be converted into an ammeter.
How will you convert a moving coil galvanometer into an ammeter?
Briefly explain why and how a galvanometer is converted into an ammeter.
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Solution 1
- A moving-coil galvanometer is transformed into an ammeter by connecting a low-resistance S across the coil and lowering its effective resistance.
- A shunt is a parallel low resistance that shunts a portion of the current around the coil, as seen in the image below. As a result, the range of currents over which the metre can be used is expanded.
Ammeter - Let I be the maximum current to be measured and Ig be the current for which the galvanometer of resistance G shows a full-scale deflection. Then, the shunt resistance S should be such that the remaining current I - Ig = IS is shunted through it.
- The potential difference across the galvanometer in the parallel combination is equivalent to the potential difference across the shunt.
∴ IgG = IsS = (I - Ig)S
∴ S = `("I"_"g"/("I" - "I"_"g"))"G"`
Solution 2
Only small currents may be detected by a galvanometer. Thus, it is transformed into an ammeter to measure enormous currents. By joining a low resistance known as a shunt resistance in parallel with the galvanometer, it can be transformed into an ammeter.
Let S be the low resistance connected in parallel with the galvanometer, G be the resistance of the galvanometer, and I be the maximum current to be measured for full-scale deflection in the galvanometer. Since the galvanometer and shunt resistances are connected in parallel.
∴ The potential difference across 'S' = Potential difference across G
(I - lg)S = lgG
Notes
Students should refer to the answer according to their questions and marks.
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